Laurie E. Drinkwater

Legume-based cropping systems have reduced carbon and nitrogen losses

In agricultural systems, optimization of carbon and nitrogen cycling through soil organic matter can improve soil fertility and yields while reducing negative environmental impact. A basic tenet that has guided the management of soil organic matter for decades has been that equilibrium levels of carbon and nitrogen are controlled by their net input and that qualitative differences in these inputs are relatively unimportant. This contrasts with natural ecosystems in which there are significant effects of species composition and litter quality on carbon and nitrogen cycling,. Here we report the net balances of carbon and nitrogen from a 15-year study in which three distinct maize/soybean agroecosystems are compared. Quantitative differences in net primary productivity and nitrogen balance across agroecosystems do not account for the observed changes in soil carbon and nitrogen. We suggest that the use of low carbon-to-nitrogen organic residues to maintain soil fertility, combined with greater temporal diversity in cropping sequences, significantly increases the retention of soil carbon and nitrogen, which has important implications for regional and global carbon and nitrogen budgets, sustained production, and environmental quality.

Nutrient Imbalances in Agricultural Development

Nutrient additions to intensive agricultural systems range from inadequate to excessive—and both extremes have substantial human and environmental costs. Nutrient cycles link agricultural systems to their societies and surroundings; inputs of nitrogen and phosphorus in particular are essential for high crop yields, but downstream and downwind losses of these same nutrients diminish environmental quality and human well-being. Agricultural nutrient balances differ substantially with economic development, from inputs that are inadequate to maintain soil fertility in parts of many developing countries, particularly those of sub-Saharan Africa, to excessive and environmentally damaging surpluses in many developed and rapidly growing economies. National and/or regional policies contribute to patterns of nutrient use and their environmental consequences in all of these situations (1). Solutions to the nutrient challenges that face global agriculture can be informed by analyses of trajectories of change within, as well as across, agricultural systems.

Nutrients in Agroecosystems: Rethinking the Management Paradigm

Agricultural intensification has greatly increased the productive capacity of agroecosystems, but has had unintended environmental consequences including degradation of soil and water resources, and alteration of biogeochemical cycles. Current nutrient management strategies aim to deliver soluble inorganic nutrients directly to crops and have uncoupled carbon, nitrogen, and phosphorus cycles in space and time. As a result, agricultural ecosystems are maintained in a state of nutrient saturation and are inherently leaky because chronic surplus additions of nitrogen and phosphorus are required to meet yield goals. Significant reductions of nutrient surpluses can only be achieved by managing a variety of intrinsic ecosystem processes at multiple scales to recouple elemental cycles. Rather than focusing solely on soluble, inorganic plant‐available pools, an ecosystem‐based approach would seek to optimize organic and mineral reservoirs with longer mean residence times that can be accessed through microbially and plant‐mediated processes. Strategic use of varied nutrient sources, including inorganic fertilizers, combined with increases in plant diversity aimed at expanding the functional roles of plants in agroecosystems will help restore desired agroecosystem functions. To develop crops that can thrive in this environment, selection of cultivars and their associated microorganisms that are able to access a range of nutrient pools will be critical. Integrated management of biogeochemical processes that regulate the cycling of nutrients and carbon combined with increased reservoirs more readily retained in the soil will greatly reduce the need for surplus nutrient additions in agriculture.

Effect of tillage and farming system upon VAM fungus populations and mycorrhizas and nutrient uptake of maize

Low-input agricultural systems that do not rely on fertilizers may be more dependent on vesicular-arbuscular mycorrhizal [VAM] fungi than conventionally managed systems. We studied populations of spores of VAM fungi, mycorrhiza formation and nutrient utilization of maize (Zea mays L.) grown in moldboard plowed, chisel-disked or no-tilled soil under conventional and low-input agricultural systems. Maize shoots and roots were collected at four growth stages. Soils under low-input management had higher VAM fungus spore populations than soils under conventional management. Spore populations and colonization of maize roots by VAM fungi were higher in no-tilled than in moldboard plowed or chisel-disked soil. The inoculum potential of soil collected in the autumn was greater for no-till and chisel-disked soils than for moldboard plowed soils and greater for low-input than conventionally farmed soil. The effects of tillage and farming system on N uptake and utilization varied with growth stage of the maize plants. The effect of farming system on P use efficiency was significant at the vegetative stages only, with higher efficiencies in plants under low-input management. The effect of tillage was consistent through all growth stages, with higher P use efficiencies in plants under moldboard plow and chisel-disk than under no-till. Plants grown in no-tilled soils had the highest shoot P concentrations throughout the experiment. This benefit of enhanced VAM fungus colonization, particularly in the low-input system in the absence of effective weed control and with likely lower soil temperatures, did not translate into enhanced growth and yield.

Comparison of substrate utilization assay and fatty acid analysis of soil microbial communities

Two methods for the characterization of microbial communities in field soil samples were compared. Experimental subplots were established in the Farming Systems Trial at the Rodale Institute Research Center in Kutztown, PA, USA. In the legume system, plots receiving green manure (hairy vetch) were compared to those receiving corn stover and a small amount of rye residue. In the conventional system, samples were collected in plots where corn stover was incorporated. Carbon source utilization profiles were developed using Biolog plates, while total soil fatty acids were determined by fatty acid methyl ester analysis. Principal component analysis and canonical discriminate analysis were used to analyze the data. The two methods gave similar but not identical results. Management history had more effect than specific crop residue, but temporal effects were greater than treatment effects. Certain shifts in microbial communities were detected by fatty acid analysis but not by carbon utilization assay, suggesting that changes in microbial species composition occurred that were not accompanied by changes in microbial community function.

Diversity of communities of arbuscular mycorrhizal (AM) fungi present in conventional versus low-input agricultural sites in eastern Pennsylvania, USA

We compared the composition and structure of the communities of arbuscular mycorrhizal (AM) fungi associated with maize (Zea maysL.) and soybeans (Glycine max(L.) Merr.) in a conventional (CON) and two low-input (LI) farming systems to better understand the relationship among AM fungi present in different agricultural systems. One LI system utilized animal manure (LI-AM) and the other green manure (LI-GM) as the nitrogen source. Spores were extracted from rhizosphere soil samples by wet-sieving to perform microscopic identification of the species and to assess frequency of occurrence. These data were used to calculate species richness, Shannon and Wiener index of diversity, and indices of dominance among other ecological measures. The results indicated that 15 consecutive years of farming under the three management practices did not cause many differences among the fungal communities. The majority of the 15 fungal species found throughout the site were present in all treatments. Sporulation of particular fungal species differed among farming systems and/or among hosts, but the general structure of AM fungal communities (according to most ecological measures) was similar for all treatments. Trap cultures were set up for the different treatments and grown for three cycles to try to recover species with low or no sporulation in natural conditions. These results also supported our conclusion about the homogeneity of the communities in the different farming system/plant host combinations, because only one species (Glomus constrictum) that was not found in the field samples sporulated in trap culture pots. Given that differences in sporulation may reflect differential rates of growth, three undescribed species plus Glomus mosseaeand Glomus etunicatumwere better established, both in the field and in trap cultures, than the other 10 species present in these soils. Also, Gigaspora gigantea accounted for more than 60% of the total volume of spores produced in each treatment, with the exception of conventional plots planted with maize where spore biovolumes were spread much more evenly among several fungal species suggesting that carbon allocation relationships were much more balanced in these plots. The focus of future studies at these sites will be a comparison of the efficacy among the communities in terms of enhancement of plant growth.

The fate of nitrogen in grain cropping systems: a meta‐analysis of 15N field experiments

Intensively managed grain farms are saturated with large inputs of nitrogen (N) fertilizer, leading to N losses and environmental degradation. Despite decades of research directed toward reducing N losses from agroecosystems, progress has been minimal, and the currently promoted best management practices are not necessarily the most effective. We investigated the fate of N additions to temperate grain agroecosystems using a meta-analysis of 217 field-scale studies that followed the stable isotope 15N in crops and soil. We compared management practices that alter inorganic fertilizer additions, such as application timing or reduced N fertilizer rates, to practices that re-couple the biogeochemical cycles of carbon (C) and N, such as organic N sources and diversified crop rotations, and analyzed the following response variables: 15N recovery in crops, total recovery of 15N in crops and soil, and crop yield. More of the literature (94%) emphasized crop recovery of 15N than total 15N recovery in crops and soil (58%), though total recovery is a more ecologically appropriate indicator for assessing N losses. Findings show wide differences in the ability of management practices to improve N use efficiency. Practices that aimed to increase crop uptake of commercial fertilizer had a lower impact on total 15N recovery (3-21% increase) than practices that re-coupled C and N cycling (30-42% increase). A majority of studies (66%) were only one growing season long, which poses a particular problem when organic N sources are used because crops recover N from these sources over several years. These short-term studies neglect significant ecological processes that occur over longer time scales. Field-scale mass balance calculations using the 15N data set show that, on average, 43 kg N x ha(-1) x yr(-1) was unaccounted for at the end of one growing season out of 114 kg N x ha(-1) x yr(-1), representing approximately 38% of the total 15N applied. This comprehensive assessment of stable-isotope research on agroecosystem N management can inform the development of policies to mitigate nonpoint source pollution. Nitrogen management practices that most effectively increase N retention are not currently being promoted and are rare on the landscape in the United States.

Effect of compost addition and crop rotation point upon VAM fungi

Populations of vesicular-arbuscular mycorrhizal (VAM) fungi and mycorrhiza formation were examined in a field experiment studying the agricultural application of composted animal manures. The replicate experiment allowed each crop of a Zea mays > vegetable > small grain rotation to be sampled each of 3 years. Chicken litter/leaf compost and dairy cow manure/leaf compost enhanced spore populations of two VAM fungus species type groups (Glomus etunicatum type and the general Glomus spp. group, including G. mosseae) relative to those found in plots treated with raw dairy cow manure and conventional fertilizer. Populations of other groups were not affected by amendment, due likely to the large amount of P added in composts and manure relative to the conventional fertilizer applied. Crop rotation point had consistent, significant effects, with both lower populations of spores and less mycorrhizal infectivity of soil in plots after the vegetable crop (Spinacea oleraceae and/or Capsicum annuum) relative to maize and small grain (Avena sativa or Triticum aestivum). This was due to the non-mycorrhizal status and very low mycorrhization (approximately 1% root length colonized) of S. oleraceae and C. annuum, respectively. Future agricultural applications of compost and manure to provide N for crops need to consider the effects upon VAM fungi of other nutrients in these amendments.

An overwintering cover crop increases inoculum of VAM fungi in agricultural soil

We conducted a field experiment within a low-input reduced tillage trial to determine how a cover crop affects inoculum levels of vesicular-arbuscular mycorrhizal (VAM) fungi. Plots with and without the hairy vetch cover crop were established on September 30, 1993, under moldboard plow (MP), chisel-disk (CD), and no-till (NT) treatments in low-input (LI) management, and MP in conventional (CONV) management. We conducted a 3-week colonization assay in the greenhouse with bahiagrass seedlings to assess the relative colonization potential of the soils in the fall and following spring. Hairy vetch roots were colonized by indigenous VAM fungi by 65 days after planting, with plants from NT being more colonized than plants from MP or CD plots. Spore populations were greater in the LI than in the CONV system. The beneficial effect of the cover crop on VAM spore populations in soil was manifested in the spring, with the Glomus type group more abundant in plots with cover than without it. The greenhouse bioassay showed that colonization potential of spring 1994 soil samples was higher in plots with cover than without cover for both the LI and CONV systems. Just one season of an overwintering cover crop of hairy vetch increased the inoculum of VAM fungi the following spring before the next cash crop was planted.

Effects of tillage intensity on nitrogen dynamics and productivity in legume-based grain systems

In 1988 an experiment was established at the Rodale Institute Experimental Farm to study weed control and nitrogen (N) management in rotations with grain crops and N-fixing green manures under reduced tillage without the use of herbicides. Tillage intensities ranging from moldboard plow (MP) to continuous no-till (NT) were compared. We present results for maize production in 1994, the seventh year of the experiment. Our goal was to further investigate reduced tillage regimes that alternated no-till with different forms of primary tillage in legume-based systems. In the chisel-disc (CD) and MP treatments comparable yields were achieved under so-called organic (weeds controlled with cultivation and green manure N source) and conventional management (weeds controlled with herbicides and mineral N fertilizer applied). Weed competition in these treatments was minimal and the N status of maize plants was essentially the same regardless of the N source (fertilizer or green manure). Of the four organic no-till maize treatments, only the mixed-tillage system with cultivation for weed control (CD-NTc) produced yields comparable to conventional NT maize. The fate of vetch N as well as temporal N dynamics were largely determined by tillage intensity and the handling of the vetch residues at maize planting. Treatments with primary tillage (CD and MP) had extremely high levels of mineral N early in the season and had greater average net N-mineralization, even though N content of hairy vetch in these treatments was equal to or lower than that in treatments with mow-killed vetch. In terms of soil mineral N concentrations, the CD-NTc treatment was similar to the other mow-killed vetch/no-till maize treatments. However, N availability in this treatment was greater, probably due to more complete decomposition of green manure residues. Cultivation for weeds not only helped control weeds but also increased mineralization of the vetch residues, which in turn increased the N supply during the period of maximum N demand by the maize. Carefully designed rotations combining tillage reductions with the use of leguminous N sources can have multiple benefits, including improved timing of N availability, reduced herbicide applications, and improved soil quality in the long term.