Direct and Indirect Impacts of Weed Management Practices ...weedeco.msu.montana.edu/publications/pdfs/Smith... · seeds, and crop residues (Buhler and Daniel, 1988; Mohler, 1993;

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18Direct and Indirect Impacts of Weed Management Practices on Soil QualityRichard G. Smith, Matthew R. Ryan, and Fabian D. Menalled

Weed management is an ever-present challenge to crop production. Weeds have the poten-tial to usurp resources that would otherwise provide nourishment to growing crops or

interfere with planting or harvesting operations. Because of these potential negative impacts, much research has been devoted to developing management strategies aimed at reducing weed populations, usually through mechanical disturbance or chemical applications (Zimdahl, 2004). Although research focused on understanding the factors that regulate crop productivity has demonstrated the linkage between soil quality, crop yield, and agricultural sustainability (Kar-len et al., 2001), the impact that weed management strategies can have on soil quality should also be considered (Fig. 18|1).

The direct and indirect effects of weed management on soil quality can range from negative to positive. Direct effects of weed management practices on soil quality have been relatively well studied (Dick, 1992; Karlen et al., 1994; Lal et al., 1994; Reeves, 1997) and several aspects of the associated soil processes are fairly well understood (Sollins et al., 1996; Robertson et al., 1999; Six et al., 2002). In contrast, the indirect effects of weed management on soil quality—effects that occur through changes in weed abundance and species composition affecting the nature and quantity of organic matter inputs, as well as feedbacks between weeds and soil biota—are much less understood. While a number of variables can be used as indicators of soil quality, for the purpose of this discussion we focus mainly on soil organic carbon as a metric of soil health because of its impact on a suite of physical, chemical, and biological properties (Cannell and Hawes, 1994; Reeves, 1997).

The goal of this chapter is several-fold. First, we explore general principles regarding direct effects of common weed management practices including mechanical practices and herbicide applications on soil carbon dynamics. In this section we highlight a weed management–soil quality paradox: although effective weed management and enhancement of soil quality are necessary components of sustainable crop production, practices focused on enhancing one com-ponent could inhibit the other. We address the weed management–soil quality paradox in the second section of this chapter by examining strategies aimed at managing both weeds and soil quality simultaneously. In the third section of the chapter, we describe a recent greenhouse

R.G. Smith, Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH 03824 ([email protected]); M.R. Ryan, Department of Crop and Soil Sciences, The Pennsylvania State University, University Park, PA 16802 ([email protected]); and F.D. Menalled, Depart-ment of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 59717 ([email protected]).

doi:10.2136/2011.soilmanagement.c18

Copyright © 2011. American Society of Agronomy and Soil Science Society of America, 5585 Guilford Road, Madison, WI 53711, USA. Soil Management: Building a Stable Base for Agriculture. Jerry L. Hatfield and Thomas J. Sauer (ed.)

276 Chapter | Authors276 Impacts of Weed Management Practices on Soil Quality | R.G. Smith et al.

study aimed at evaluating potential indirect effects of weed management on soil quality and crop growth. Finally, we discuss recent applied agroecological research focused on developing weed-suppressive soils and agroecosystems, and offer suggestions for future research.

Direct Effects of Major Weed Management Practices on Soil CarbonTillage Systems and Soil DisturbancePerhaps the most consequential change in soil quality associated with weed manage-ment occurs immediately after uncultivated land is put into production and mechan-ical management practices aimed at reducing weed abundance are implemented. For example, following initial cultiva-tion, soil organic carbon can decrease by 30% relative to its precultivated condition (Davidson and Ackerman, 1993). The effects of subsequent cultivations on soil carbon dynamics generally depend on the nature of the implements, the severity of disturbance, and the condition of the soil at the time of implementation (Bowman, 1997; Franzlueb-bers et al., 1999; Mohler, 2001; Barberi, 2002; Koch and Stockfisch, 2006).

As a weed management strategy, tillage can be divided into three main categories: primary, secondary, and ter-tiary. Mohler (2001) provides a comprehensive overview of the different types and modes of action of various mechani-cal implements. The goal of primary tillage is to reduce the abundance, competi-tive ability, and reproductive potential of emerged weeds. Implements used for pri-mary tillage vary in their intensity of soil disturbance and efficacy for weed con-trol. Moldboard plow tillage is considered full-inversion tillage and results in the most physically intensive distur-bance to the soil (Nichols and Reed, 1934). Inversion of the soil results in burial and

redistribution of emerged weeds, weed seeds, and crop residues (Buhler and Daniel, 1988; Mohler, 1993; Buhler 1995; Clements et al., 1996). Less intensive forms of primary tillage, such as chisel plow tillage, result in reduced disturbance to the soil profile, but can also be less effective as a weed control practice. As a general rule, higher weed bio-mass is associated with chisel plow tillage compared with moldboard plow tillage (Fig. 18|2), demonstrating the tradeoff between weed suppression and reduced physical dis-turbance to the soil (Buhler and Oplinger, 1990; Ryan, unpublished data, 2002).

Secondary tillage is used to prepare the seedbed for planting. Implements used in secondary tillage include disks, rotova-tors, and harrows. As a weed management tool, secondary tillage can be important for reducing weed seeds that would otherwise germinate with the next crop. This practice, termed stale (or false) seedbed preparation, is often repeated several times to maximize the potential for reduction of the germinable weed seed bank (Mohler, 2001). Soil distur-bances associated with secondary tillage are often confined to the upper 10 cm of the soil, and typically result in reduced impacts on soil organic carbon relative to practices that disturb the soil to a greater depth. For example, a loess soil maintained under a conservation tillage program in Germany

Fig. 18|1. Conceptual model of the direct and indi-rect effects of weed management on soil quality. Direct effects are primarily mediated through physical and chemical processes. Indirect effects are mediated through changes in weed abundance and species composition and their impacts on soil chemical and bio-logical processes.

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where annual soil mixing was restricted to a depth of 10 cm contained 6% more soil organic carbon at the 0- to 46-cm depth than the same soil subjected to just three mold-board plow tillage events conducted over 3 yr (Koch and Stockfisch, 2006).

Tertiary tillage or cultivation involves direct interference with germinating or emerged weeds and it can be performed before or after crop emergence. Tertiary till-age implements range from scraping the soil surface to uproot recently emerged weed seedlings (tine weeder), to destroying weeds in the interrows (S-tine cultivator), to cutting the roots of weeds below the soil surface with minimal disturbance to the soil profile (lister or high residue cultivator set at a low angle with wide V-shaped sweeps), to transferring soil from the interrows to bury weed seedlings growing in the crop rows (concave discs and shovels). Tertiary tillage results in variable levels of soil dis-turbance and weed suppression depending on the implement. For example, when using tertiary cultivation equipment, weeds can recover and persist if soil clods are not ade-quately disturbed (Bond and Grundy, 2001).

No-Tillage Cropping Systems, Herbicide, and Soil QualityThe wide availability of syn-thetic herbicides in the last 60 years has made it possible to reduce mechanical approaches to weed control and increased adoption of reduced and no-tillage crop production. The increased interest in reduced and no-tillage practices has also been driven by growing under-standing of the detrimental effects of mechanically based weed control approaches on soil quality, particularly in regions susceptible to soil erosion (Geb-hardt et al., 1985).

Reducing tillage intensity can have positive impacts on soil carbon dynamics, the formation rate of soil aggregates, water infiltration patterns, soil mois-ture retention, and the potential for soil crusting (Karlen et al., 1994; Six et al., 1999; Tebrügge

and Düring, 1999; Wander and Bollero, 1999; Cambardella et al., 2004). Reduction or elim-ination of tillage has been associated with a shift in weed communities (Menalled et al., 2001) and increases in populations of agroecologically important soil macroinver-tebrates, such as earthworms (Karlen et al., 1994; Smith et al., 2008a) and ground bee-tles (Menalled et al., 2007), which can have additional direct impacts on soil quality and weed population dynamics (Smith et al., 2005; Menalled et al., 2007).

While it is clear that reduced and no-till-age systems can have beneficial impacts on soil quality parameters, the heavy reliance on synthetic herbicides creates management challenges. Apart from the increased pres-sure to select for herbicide-resistant weed biotypes (Heap, 2010) and their nontarget impacts on water quality and human health (Liebman, 2001; Gilliom et al., 2006), herbi-cides can also have unintended effects on soil quality parameters (Roper and Gupta, 1995). These effects occur primarily through changes in the quality, quantity, and diver-sity of organic inputs into the soil from weed biomass (Hipps and Samuelson, 1991).

Fig. 18|2. Impact of seedbed preparation method and presence and type of cover crop on weed abundance from a study conducted at the Rodale Institute in Kutz-town, Pennsylvania. Weed biomass was significantly higher in the chisel plow tillage plots compared with moldboard plow tillage plots (P < 0.001), whereas there was no effect from the cover crop combina-tion (P < 0.9423), or the interaction between tillage and cover crop combinations (P < 0.8437). Data are pooled across years and bars represent the standard error of the mean.


Evidence for direct effects of repeated herbicide applications on soil microbial com-munities and their controls on soil quality have, to this point, been equivocal. In a recent review, Bünemann et al. (2006) reported few significant negative effects of herbicides on soil organisms. Similarly, Hartley et al. (1996) indicated that there were no effects of a resid-ual herbicide (terbuthylazine) on in situ soil respiration or cellulose degradation when compared with several organic weed man-agement practices. However, Santos et al. (2006) suggests that the effects of herbicides on soil microbial communities may depend on tillage systems. Furthermore, Zilli et al. (2008) found that while glyphosate and imazaquin-based herbicides had no effect on soil microbial biomass carbon, basal res-piration, and metabolic quotient, there were changes in the bacterial profile in the rhizo-plane, suggesting that herbicide applications may alter the structure of soil microbial com-munities. Similarly, when examining the impact of bromoxynil on soil-derived micro-bial communities, Baxter and Cummings (2006) found that the application of the her-bicide exerted deterministic selection on the microbial community. Finally, in a critique of studies evaluating the effects of glyphosate on diseases associated with Fusarium species, Powell and Swanton (2008) indicate the need for further research aimed at determining if there are indeed direct negative effects on soil health from glyphosate applications.

Weed Management Coupled with Soil-Building PracticesOrganic Matter InputsThe deleterious effects of common mechani-cal and chemical-based weed management practices on soil organic carbon may be offset by soil-building practices such as applying manure or compost and growing cover crops. For example, in the long-term Rodale Institute Farming Systems Trial in Kutztown, Pennsylvania, two organic crop management systems were compared with a conventional system to assess the effects of management systems on various agroeco-system properties. The two organic systems received no applications of synthetic fertil-izer or pesticides and included treatments

with two different kinds of organic matter inputs: (i) organic animal-based (a diverse rotation including corn, soybeans, corn silage, wheat, and red clover–alfalfa hay, as well as a rye cover crop before corn silage and soybeans, with aged cattle manure applied as a supplemental nitrogen and organic matter source), and (ii) legume cover crop-based (a 5-yr rotation of hairy vetch [winter cover crop used as a green manure], corn, rye [winter cover crop], soybeans, and winter wheat). The conventional system (a 5-yr corn–corn–soybean–corn–soybean rotation) was managed with fertilizer and pesticide applications following Pennsylva-nia State University Cooperative Extension recommendations (Pimentel et al., 2005). Moldboard plow tillage and cultivation were performed four to eight times in any given cropping season in the organic sys-tems and chisel plow tillage was used in the conventional system. Despite the inten-sive rate of soil disturbances applied in the organic systems, significantly higher soil microbial biomass, soil respiration, and arbuscular mycorrhizal fungi popula-tions were reported in the organic systems than in the conventional system (Douds et al., 1993; Wander et al., 1994; Harris et al., 1994; Douds and Millner, 1999). Similarly, results from a 9-yr cropping systems trial comparing organic, no-tillage, and conven-tional tillage cropping systems, showed that the organic systems with cover crops and manure achieved higher levels of soil car-bon, despite the use of more intensive tillage and cultivation (Teasdale et al., 2007).

Reduced Tillage in Low- and Organic-Input SystemsThe practices of reduced and no-tillage have been restricted to systems in which high rates of herbicides can be applied. However, recent agricultural and techno-logical developments may enable growers to manage weeds in reduced tillage sys-tems with minimum or no herbicide inputs. Organic amendments and crop diversifica-tion through rotation and cover crops are important components of strategies aimed at improving soil quality while managing weeds in reduced- or no-herbicide systems (Liebman and Dyck, 1993; Liebman and Davis, 2000). For example, winter cereal cover crops can be effective components of

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integrated weed management systems that rely on lower herbicide rates in reduced till-age systems (Price et al., 2006). Cereal rye (Secale cereale L.) is a popular winter cereal cover crop in the northeast region, mainly because of its cold hardiness and ability to produce large amounts of biomass (~10,000 kg/ha), an attribute positively associated with weed suppression (Teasdale, 1996). Leguminous cover crops such as red clo-ver (Trifolium pratense L.) and crimson clover (Trifolium incarnatum L.) can also suppress weeds in reduced-input and organic sys-tems through competitive and allelopathic effects, provide habitat to seed-predating macroinvertebrates, and contribute to soil fertility and organic carbon (Davis and Lieb-man, 2003, Smith et al., 2008b).

Constraints to implementing reduced or no-tillage in low-input or organic sys-tems include difficulty in terminating the cover crops before planting cash crops and managing weeds (Peigne et al., 2007). Development of new cover crop manage-ment systems, such as roller-crimpers may alleviate some of the constraints associated with cover crops, though these are still in their infancy. Depending on the design of the roller, crimped cover crop termination rates can range from 83% to 94% (Kornecki et al., 2006), and the residue provides a weed-suppressing layer of mulch that can reduce the need for cultivation.

An alternative approach to continuous no-tillage in reduced- and organic-input cropping systems could be to rotate till-age—necessary for weed management during some phases of the rotation—with no-tillage in other phases of the rotation when weed pressures are low (Peigne et al., 2007). This would allow the soil-building benefits of reduced tillage and organic mat-ter inputs to be realized over some phases of the rotation, which might help offset the potential negative effects associated with more intensive tillage during particularly weed-sensitive phases of the rotation.

Indirect Effects of Weed Management on Soil EcosystemsUnderstanding how plants alter the soil ecosystem is critical to the management of

crop–weed interactions and the consequent impacts of weed management practices on soil quality. Weeds are often perceived as exerting only negative effects on crop growth through competition for limiting soil nutrients, water, and light, thus necessitat-ing management practices aimed at reducing these effects. While there is ample evidence to support this perception (Knezevic et al., 2002; Zimdahl, 2004), there is also an increas-ing body of theory (reviewed in Tilman and Lehman, 2001; Hooper et al., 2005; Smith et al., 2010) and experimental evidence (Reyn-olds et al., 2003; Zak et al., 2003; Wardle et al., 2004; Jordan and Vatovec, 2004; Hooper et al., 2005; Pollnac et al., 2009) suggesting that the specific characteristics of the weed commu-nity (i.e., species composition and diversity of the weeds growing in a field) may affect the nature of crop–weed interactions, potentially offsetting or compounding the competitive effects of weeds on crops.

The diversity, composition, and abun-dance of weed communities could impact the competitive effects of weeds on crops and soil ecosystems through two potential pathways: (i) complementary resource use or facilitative interactions among co-occur-ring plant species (Hille Ris Lambers et al., 2004), and (ii) by influencing the composi-tion and functioning of the soil microbial community, including the abundance of pathogenic and beneficial microorganisms (Bever et al., 1997; Wardle et al., 2003). The first of these two pathways implies that increased diversity could result in greater total resource acquisition in diverse than in simple (low species diversity) weed com-munities. In grassland ecosystems, many of the effects of plant diversity on ecosys-tem function are thought to be mediated by complementary resource use among spe-cies that differ in their requirements for and modes of acquiring limiting resources (Tilman et al., 1996; Hector et al., 1999; Til-man and Lehman, 2001). An important prediction of the resource-use (or niche) complementarity hypothesis is that plant communities containing a high diversity of plant species should have fewer resources left unexploited compared with communi-ties containing fewer species (Elton, 1958; McNaughton, 1993; Naeem et al., 1994; Til-man et al., 1996; Tilman and Lehman, 2001). This suggests that in agricultural systems, for a given weed density, diverse weed


communities might take up a greater quan-tity and diversity of soil resources compared with simple weed communities, affecting crop resource availability and the quantity and nature of soil organic inputs through root exudates and biomass turnover. To our knowledge, there have been few direct tests of this hypothesis (Tilman et al., 2001), especially within the context of weeds in managed agricultural systems. In a manip-ulative study assessing the joint impact of species density and species diversity on crop yield, Pollnac et al. (2009), determined that species richness had no effect on spring wheat biomass, yield, or relative growth rate, and that there was no evidence for reduced competitive effects on crop plants due to interspecific competition between neighbor weed species. However, weed species rich-ness had a negative effect on the growth of individual weed species and competitive ability of dominant weed species.

The second pathway by which weed diversity may impact the soil ecosystem and weed–crop competition is by influenc-ing the composition and functioning of the soil microbial community, including the abundance of pathogenic and beneficial microorganisms. Changes in the soil micro-bial community following plant invasion have been reported in a number of non-agricultural systems (Kourtev et al., 2003; Callaway et al., 2004; Hawkes et al., 2005; Wolfe and Klironomos, 2005). Changes in the microbial community mediated by shifts in plant community composition and diver-sity can then lead to additional feedbacks to the plant community (Bever, 1994; Bever et al., 1997; Klironomos, 2002; Reynolds et al., 2003; Clark et al., 2005; Mikola et al., 2005; Wolfe and Klironomos, 2005). These feed-backs are mediated through changes in decomposition rates (Hobbie, 1992; Knops et al., 2002), resource availability (Vinton and Burke, 1995; Ehrenfeld et al., 2001; Zak et al., 2003; Clark et al., 2005) and pathogenic and mutualistic interactions (Hamel et al., 2005; Wolfe and Klironomos, 2005).

In agricultural systems, it is known that continuously planting the same crop can lead to increased soil pathogen loads, which can severely decrease crop perfor-mance (Bruehl, 1987). Understanding this crop–pathogen link has lead to manage-ment practices such as crop rotation aimed at disrupting pathogen population build-up

(Bullock, 1992; Liebman and Dyck, 1993; Hamel et al., 2005). However, the poten-tial for arable weeds to affect soil microbial communities in agroecosystems is less well understood (Li and Kremer, 2000). Changes in soil microbial communities driven by weed community composition and diver-sity could potentially lead to either positive or negative feedbacks on crop growth and competitive ability, depending on the weed-species specificity and the nature (positive or negative) of the feedback. Also unknown, is the degree to which changes in soil resource availability due to weed diversity are medi-ated through changes in the soil community (but see Zak et al., 2003). Here we present data from a recent greenhouse study aimed at elucidating the pathways mediating indi-rect effects of weeds on crops.

The experiment was conducted with field soils collected from an organic small grain–legume rotation near Bozeman, Montana that had been managed without synthetic inputs for 12 yr (Maxwell et al., 2007). The soil was collected from the field in fall 2005 and mixed into a blend that contained organ-ically managed field soil, mineral soil, sand, and peat (1.0 part: 0.5 parts: 1.25 parts: 0.25 parts, respectively). The soil mixture was then subjected to two treatments: steam pasteurization at 70°C for 2 h to reduce the activity of the naturally occurring microbial community (hereafter SP), and no steam pas-teurization (hereafter NSP). Each of the two soil mixtures were then used to fill large pots (25.4-cm diameter, 35.0-cm height) into which different weed community treatments were sown in a three-phase design, with each treatment replicated eight times. The goal of the first two phases was to prime soils with plant communities differing in plant species composition and diversity. The goal of the last phase was to assess the response of wheat to these primed soils. Species used in this study included five agriculturally important plant species, four weed species—wild oat (Avena fatua L.), green foxtail [Setaria viridis (L.) P. Beauv.], field pennycress (Thlaspi arvense L.), and redroot pigweed (Amaranthus retroflexus L.)—and one crop, spring wheat var. McNeal (Triticum aestivum L.). The reasoning support-ing this study was that differences in wheat biomass in the third phase between soil pasteurization treatments and across plant communities would indicate the importance of each potential pathway (simple resource

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uptake or changes in soil communities) in mediating the observed responses (Fig. 18|3).

In the first two phases, each plant species was sown in monoculture, all possible three-species combinations, and in a mixture that contained all five species. Pots were watered with a drip irrigation system, as needed and plant communities were thinned to a den-sity of 15 individuals following emergence (multispecies communities were thinned in such a way as to maintain an equivalent proportion of each species within the com-munity). Plant communities were grown for ~40 d, after which time all aboveground biomass was removed, dried at 60°C, and weighed. This resulted in two “generations” of soil priming. Approximately 10 d after the second harvest, the third phase was ini-tiated and wheat was planted into all pots to

measure its response to the previous plant community treatments. Wheat populations in each pot were thinned to 15 individuals and the biomass was harvested after 40 d, dried, and weighed.

The three main results obtained in this study expand our understanding of the potential importance of interactions between weed and soil communities. First, the native soil community appeared to exhibit inhibitory control on the growth of each of the five plant species examined. By pasteurizing the soil, and reducing the activ-ity of the native soil microorganisms, plant growth increased for each of the five species (Fig. 18|4). Second, while we observed a sig-nificant negative relationship between the diversity of the plant community in the first two phases of the study and wheat biomass

Fig. 18|3. Theoretical framework for interpreting plant response to manipulations of weed diversity. Responses observed in soil containing a naturally occurring microbial community (NSP) are compared with those observed in soil in which the naturally occurring microbial community has been minimized by means of steam pasteurization (SP).

Fig. 18|4. Growth of five agricultur-ally important plant species in soil con-taining a naturally occurring microbial community (left pot in each set) and soil in which the micro-bial community has been minimized by steam pasteuriza-tion (SP).


in the third phase of the study in the NSP treatment, no relationship between plant diversity and wheat biomass was observed in the SP treatment (Fig. 18|5). The fact that in the SP treatment we reduced microbial activity, but did not, in theory, alter resource availability, leads us to conclude that weed diversity reduced the growth of wheat pri-marily through impacts on soil communities rather than by simple nutrient uptake. Third, individual weed species differed in their effects on soil community-mediated feed-backs on wheat growth, and these effects depended on soil microbial activity (Fig. 18|6). In the NSP treatment, wheat biomass in the third phase of the study was lowest when wheat followed wheat, intermediate when it followed green foxtail, and highest when it followed redroot pigweed. In con-trast, there was no relationship between the

identity of the previous weed species and the growth of wheat in the SP treatment. Soil nitrate was measured before sowing wheat in the third phase of the study and indicated that differences in resource avail-ability between the five species at the end of the second phase were not related to wheat biomass response in phase three, further suggesting that the observed response in wheat was driven by microbial-related feed-backs (data not shown).

Future Directions and Necessary ResearchA better understanding of the complex interactions that can occur between weeds and soil communities may allow us to develop strategies for managing weeds

Fig. 18|5. Response of wheat to the number of plant species grown in a pot for two generations before the planting of wheat in soils containing a naturally occurring micro-bial community (NSP) and in which the naturally occurring microbial com-munity was reduced by steam pasteurization (SP).

Fig. 18|6. Response of wheat to the identity of the previous plant grown for two genera-tions in soils containing a naturally occurring microbial community (NSP) and in which the naturally occurring microbial community was reduced by steam pasteurization (SP). Data are means ± SE, N = 8. Bars sharing the same letter are not significantly different at P < 0.05 (Fisher’s LSD test).

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while reducing direct and indirect negative impacts on soil quality (Kremer and Li, 2003; Smith et al., 2010). Managing soils in ways that make soils more resistant to weedy invasions or increase the abundance of weed-suppressive or antagonist microbes or other soil organisms is a promising avenue of research that has until now elicited only limited investigation (Kremer, 1993; Kremer and Li, 2003; Gallandt et al., 2004; Chee-San-ford et al., 2006; Davis et al., 2006). Although in a recent review, Vasquez et al. (2008) pro-posed that adequate nitrogen management can help create more invasion-resistant soils, more work is needed to understand how weedy invasions are mediated through the soil fungal and bacterial community and to identify the specific components of the soil communities that are important deter-minants of weed growth, fitness, and seed degradation (Davis et al., 2006).

It is also likely that greater progress will be made when research explicitly addresses strategies that integrate multiple tactics (Liebman and Gallandt, 1997). For example, combines can be modified to capture weed seeds during crop harvest to reduce weed populations in the soil seed bank (Davis et al., 2005). This tactic combined with other cultural practices, such as planting competi-tive cultivars or enhancement of weed seed predator populations through the establish-ment of noncrop habitat around crop fields (Landis et al., 2000), may result in enough cumulative effects on weed populations to reduce the necessity for intensive soil dis-turbances. Unfortunately, the majority of crop breeding programs have been per-formed to maximize growth response to fertilizer application, and in the absence of weed competition. Breeding efforts aimed at improving crop competitiveness in the presence of weeds could reduce the need for employing potentially soil-damaging management practices (Christensen, 1995; Pester et al., 1999; Lemerle et al., 2001; Olof-sdotter et al., 2002). To date, there has been surprisingly little work in this arena, and opportunities to improve integrated weed management through competitive cultivar selection abound.

Lastly, because of the negative impact of weeds on crop yield and quality, soil-condi-tioning benefits of weedy plants are seldom discussed (Sturz et al., 2001). However, recent research suggests that crop plants may

benefit from noncrop plants growing in their presence either at low densities or at limited times throughout the growing season (Sturz et al., 2001; Jordan and Vatovec, 2004). Poten-tial soil quality benefits provided by weedy plants include: (i) sequestering carbon, (ii) cycling nutrients, (iii) hosting mycorrhizal fungi populations, and (iv) supporting plant-growth-promoting rhizobacteria. Other ecosystem services enhanced from increas-ing benign noncrop vegetation in arable landscapes include insect pest suppression, buffering of microclimate and hydrologic processes, providing cover and habitat, and detoxification of noxious chemicals (Alt-ieri, 1999; Gerowitt et al., 2003; Marshall et al., 2003). Research focused on developing a better understanding of the potential benefits that weeds may provide to cropping systems could illuminate opportunities to enhance their usefulness, reduce their negative impacts on crop yields, and improve strate-gies to manage their populations in ways that are beneficial to soil quality.

ConclusionsA better understanding of the direct and indirect relationships between weed man-agement practices and soil quality is a necessary step to sustain agricultural pro-ductivity in the face of a growing global population and a finite abundance of nat-ural resources. Problems of erosion of the world’s agricultural soils provide strong evidence of the tight coupling between agricultural management practices and soil quality (Montgomery, 2007). As pub-lic awareness grows regarding where and how food is produced, so too does the need for agricultural scientists to provide grow-ers and land managers the tools they need to grow that food profitably and with mini-mal impact on the environment (Robertson and Swinton, 2005). An ecologically based approach to agroecosystem management relies on a keen understanding of processes that affect the weed populations and com-munity dynamics and increase the relative competitive ability of crops. As the research reviewed here suggests, there may be oppor-tunities to tip the balance between weeds and crops in ways that favor the crop, and that do not necessarily come at the expense of soil quality.


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Direct and Indirect Impacts of Weed Management Practices ...weedeco.msu.montana.edu/publications/pdfs/Smith... · seeds, and crop residues (Buhler and Daniel, 1988; Mohler, 1993;