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Successful modeling of the carbon (C) cycle requires empirical data regarding species‐specific root responses to edaphic characteristics. We address this need by quantifying annual root production of three bioenergy systems (continuous corn, triticale/sorghum, switchgrass) in response to variation in soil properties across a toposequence within a Midwestern agroecosystem. Using ingrowth cores to measure annual root production, we tested for the effects of topography and 11 soil characteristics on root productivity. Root production significantly differed among cropping systems. Switchgrass root productivity was lowest on the floodplain position, but root productivity of annual crops was not influenced by topography or soil properties. Greater switchgrass root production was associated with high percent sand, which explained 45% of the variation. Percent sand was correlated negatively with soil C and nitrogen and positively with bulk density, indicating this variable is a proxy for multiple important soil properties. Our results suggest that easily measured soil parameters can be used to improve model predictions of root productivity in bioenergy switchgrass, but the edaphic factors we measured were not useful for predicting root productivity in annual crops. These results can improve C cycling modeling efforts by revealing the influence of cropping system and soil properties on root productivity.

Fractions of soil organic matter (SOM) were extracted by an integrated physical–chemical procedure and their chemical natures were characterized through 13C nuclear magnetic resonance (NMR) spectroscopy. For the 0- to 5-cm depth of a corn (Zea mays L.)–soybean [Glycine max. (L.) Merr.] soil in Iowa, we extracted in sequence the light fraction, two size fractions of particulate organic matter (POM), and two NaOH-extractable humic acid fractions based on their binding to soil Ca2+: the unbound mobile humic acid fraction and the calcium humate fraction. Whole SOM was obtained by dissolving the soil mineral component through HF washes. All samples were analyzed by advanced 13C NMR techniques, including quantitative direct polarization/magic angle spinning, spectral-editing techniques, and two-dimensional 1H–13C heteronuclear correlation NMR. The NMR spectra were comparable for the light fraction and two POM fractions and were dominated by carbohydrates and to a lesser extent lignins or their residues, with appreciable proteins or peptides. By contrast, spectra of the two humic fractions were dominated by aromatic C and COO/N–C=O groups, with smaller proportions of carbohydrates and NCH/OCH3 groups, indicative of more humified material. This trend was yet more pronounced in the calcium humate fraction. The spectrum for whole SOM had signals intermediate between these two groups of SOM fractions, suggesting contributions from both groups. Our results for this soil suggest that either chemical or physical fractions alone will partially represent whole SOM, and their integrated use is likely to provide greater insight into SOM structure and possibly function, depending on the research issue.

Understanding how the quality of organic soil amendments affects the synchrony of nitrogen (N) mineralization and plant N uptake is critical for optimal agronomic N management and environmental protection. Composting solid livestock manures prior to soil application has been promoted to increase N synchrony; however, few field tests of this concept have been documented. Two years of replicated field trials were conducted near Boone, Iowa to determine the effect of composted versus fresh solid swine manure (a mixture of crop residue and swine urine and feces produced in hoop structures) on Zea mays (maize) N uptake, in situ soil net N mineralization, and soil inorganic N dynamics. Soil applications of composted manure increased maize N accumulation by 25 % in 2000 and 21 % in 2001 compared with fresh manure applications (application rate of 340 kg N ha −1 ). Despite significant differences in net N mineralization between years, within year seasonal total in situ net N mineralization was similar for composted and fresh manure applications. Partial N budgets indicated that changes in soil N pools (net N mineralization and soil inorganic N) in the surface 20 cm accounted for 67 % of the total plant N accumulation in 2000 but only 16 % in 2001. Inter-annual variation in maize N accumulation could not be attributed to soil N availability. Overall, our results suggest that composting manures prior to soil application has no clear benefit for N synchrony in maize crops. Further work is required to determine the biotic and abiotic factors underlying this result.

Tillage intensity can affect chemical soil health indicators either positively or negatively depending on inherent soil properties and management practices. Soil chemical data (total N, P, K, Ca, Mg, and pH) from four depth increments within 196 published studies were compiled and subjected to a meta‐analysis comparing chisel plow (CP), no‐till (NT), and perennial systems (PER) with moldboard plowing (MP). Overall, CP did not affect soil chemical indicators when compared to MP, but converting from MP to NT increased total N, P, and K concentrations within the top 15 cm. Below that depth, Ca and Mg concentrations were lower under NT than MP but total N, P, and K were not significantly different. Topsoil total N response to NT was moderated by soil order and cropping system, with the largest increase in total N found in Ultisols, Inceptisols, Alfisols, and Mollisols under more diversified cropping systems including those with cover crops. The greatest topsoil P increase in response to NT was found under long‐term experiments (>5‐yr) and on fine‐textured soils. Phosphorus changes in studies on coarse‐textured soils, with short‐term duration, and manure applications were generally neutral. Perennial systems had lower soil P and K but higher total N content in the surface layer as compared to MP. The positive response to PER systems was most notable in Alfisols, Mollisols, and Ultisols and under long‐term PER management. Finally, we demonstrate that these chemical indicators respond to tillage and cropping systems over a wide range of conditions, showing utility for soil health assessment.

Afforestation of degraded cropland can sequester atmospheric C; however, source partitioning and turnover of soil organic C (SOC) in such ecosystems are not well documented. This study assessed SOC dynamics in two 35‐yr‐old, coniferous afforestation sites (i.e., a forest plantation situated in northwestern Iowa on a silty clay loam soil and a shelterbelt situated in eastern Nebraska on a silt loam soil) and the adjacent agricultural fields. Soil samples were collected at both sites to determine SOC and total N concentrations and stable C isotope ratios (δ13C, natural abundance) in both whole soil and the fine particulate organic matter (POM) fraction (53–500 μm size). In these fine‐textured soils, afforestation of cropland performed through either shelterbelt or forest plantation caused substantial increases in surface SOC storage compared with conventionally tilled cropping systems (≥57%; P < 0.05); this confirms the direct benefits of tree planting on SOC sequestration. Relative to cropped soils, afforested soils exhibited a more depleted δ13C signature (−17 vs. −22‰), indicating a shift in C sources. Source‐partitioning assessment revealed that tree‐derived C contributed roughly half of the SOC found directly beneath the trees. The C‐enriched afforested surface soils exhibited SOC turnover rates of 0.018 to 0.022 yr−1 and mean residence times of 55 to 45 yr. Fine POM in afforested surface soils accounted for a large proportion (21%) of the existing SOC, 79% being derived from tree inputs. This supports the role of POM as a significant sink for recently sequestered SOC in these ecosystems.

High‐diversity mixtures of perennial tallgrass prairie vegetation could be useful biomass feedstocks for marginal farmland in the Midwestern United States. These agroenergy crops can help meet cellulosic agrofuel targets while also enhancing other ecosystem services on the landscape. One proposed advantage of high‐diversity prairie biomass feedstocks is that they should become nutrient limited at a slower rate than monoculture feedstocks. In this study, we examine rates of soil nutrient depletion and the physiology and performance of a focal species (switchgrass, Panicum virgatum L.) in four prairie agroenergy feedstocks with different species composition and diversity. The feedstocks in this study were a 1‐species switchgrass monoculture, a 5‐species mixture of C4 grasses, a 16‐species mixture of C3 and C4 grasses, forbs, and legumes, and a 32‐species mixture of C3 and C4 grasses, forbs, legumes, and sedges. To assess feedstock effects on soil, we measured changes in soil N/P/K over a five‐year period. We also performed a greenhouse study, in which we grew switchgrass plants in field soil conditioned by each feedstock. To assess feedstock effects on plant function, we measured four physiological traits (photosynthetic rate, chlorophyll concentration, leaf florescence, leaf N concentration) on switchgrass plants within each feedstock in the field. In the soil analysis, we found that the 5‐species feedstock displayed higher rates of soil N/P/K depletion than other feedstocks. In the greenhouse analysis, we found that switchgrass plants grown in soil conditioned by the 5‐species feedstock were smaller than plants grown in soil conditioned by other feedstocks. In the physiological analysis, we found that switchgrass plants in the 5‐species feedstock had lower leaf N, photosynthesis, chlorophyll concentration, and higher florescence than switchgrass plants growing in other feedstocks. Collectively, our results show that prairie agroenergy feedstocks with different species composition and diversity have different rates of soil nutrient depletion, which influences the physiology and performance of plants within the feedstock. These differences would ultimately impact the ecosystem services (e.g., biomass production, need for fertilizer) that these prairie agroenergy feedstocks provide.

Our objective is to provide an optimistic strategy for reversing soil degradation by increasing public and private research efforts to understand the role of soil biology, particularly microbiology, on the health of our world’s soils. We begin by defining soil quality/soil health (which we consider to be interchangeable terms), characterizing healthy soil resources, and relating the significance of soil health to agroecosystems and their functions. We examine how soil biology influences soil health and how biological properties and processes contribute to sustainability of agriculture and ecosystem services. We continue by examining what can be done to manipulate soil biology to: (i) increase nutrient availability for production of high yielding, high quality crops; (ii) protect crops from pests, pathogens, weeds; and (iii) manage other factors limiting production, provision of ecosystem services, and resilience to stresses like droughts. Next we look to the future by asking what needs to be known about soil biology that is not currently recognized or fully understood and how these needs could be addressed using emerging research tools. We conclude, based on our perceptions of how new knowledge regarding soil biology will help make agriculture more sustainable and productive, by recommending research emphases that should receive first priority through enhanced public and private research in order to reverse the trajectory toward global soil degradation.

Erosion rates and annual soil loss tolerance (T) values in evaluations of soil management practices have served as focal points for soil quality (SQ) research and assessment programs for decades. Our objective is to enhance and extend current soil assessment efforts by presenting a framework for assessing the impact of soil management practices on soil function. The tool consists of three steps: indicator selection, indicator interpretation, and integration into an index. The tool's framework design allows researchers to continually update and refine the interpretations for many soils, climates, and land use practices. The tool was demonstrated using data from case studies in Georgia, Iowa, California, and the Pacific Northwest (WA, ID, OR). Using an expert system of decision rules as an indicator selection step successfully identified indicators for the minimum data set (MDS) in the case study data sets. In the indicator interpretation step, observed indicator data were transformed into unitless scores based on site-specific algorithmic relationships to soil function. The scored data resulted in scientifically defensible and statistically different treatment means in the four case studies. The efficacy of the indicator interpretation step was evaluated with stepwise regressions using scored and observed indicators as independent variables and endpoint data as iterative dependent variables. Scored indicators usually had coefficients of determination (R2) that were similar or greater than those of the observed indicator values. In some cases, the R2 values for indicators and endpoint regressions were higher when examined for individual treatments rather than the entire data set. This study demonstrates significant progress toward development of a SQ assessment framework for adaptive soil resource management or monitoring that is transferable to a variety of climates, soil types, and soil management systems.

Corn (Zea mays L.) stover is a potential bioenergy feedstock, but little is known about the impacts of reducing stover return on yield and soil quality in the Northern US Corn Belt. Our study objectives were to measure the impact of three stover return rates (Full (~7.8 Mg ha−1 yr−1), Moderate (~3.8 Mg ha−1 yr−1) or Low (~1.5 Mg ha yr−1) Return) on corn and soybean (Glycine max. L [Merr.]) yields and on soil dynamic properties on a chisel-tilled (Chisel) field, and well- (NT1995) or newly- (NT2005) established no-till managed fields. Stover return rate did not affect corn and soybean yields except under NT1995 where Low Return (2.88 Mg ha−1) reduced yields compared with Full and Moderate Return (3.13 Mg ha−1). In NT1995 at 0–5 cm depth, particulate organic matter in Full Return and Moderate Return (14.3 g kg−1) exceeded Low Return (11.3 g kg−1). In NT2005, acid phosphatase activity was reduced about 20% in Low Return compared to Full Return. Also the Low Return had an increase in erodible-sized dry aggregates at the soil surface compared to Full Return. Three or fewer cycles of stover treatments revealed little evidence for short-term impacts on crop yield, but detected subtle soil changes that indicate repeated harvests may have negative consequences if stover removed.

A single ecosystem dominates the Midwestern United States, occupying 26 million hectares in five states alone: the corn–soybean agroecosystem [Zea mays L.-Glycine max (L.) Merr.]. Nitrogen (N) fertilization could influence the soil carbon (C) balance in this system because the corn phase is fertilized in 97–100% of farms, at an average rate of 135 kg N·ha−1·yr−1. We evaluated the impacts on two major processes that determine the soil C balance, the rates of organic-carbon (OC) inputs and decay, at four levels of N fertilization, 0, 90, 180, and 270 kg/ha, in two long-term experimental sites in Mollisols in Iowa, USA. We compared the corn–soybean system with other experimental cropping systems fertilized with N in the corn phases only: continuous corn for grain; corn–corn-oats (Avena sativa L.)–alfalfa (Medicago sativa L.; corn–oats–alfalfa–alfalfa; and continuous soybean. In all systems, we estimated long-term OC inputs and decay rates over all phases of the rotations, based on long-term yield data, harvest indices (HI), and root : shoot data. For corn, we measured these two ratios in the four N treatments in a single year in each site; for other crops we used published ratios. Total OC inputs were calculated as aboveground plus belowground net primary production (NPP) minus harvested yield. For corn, measured total OC inputs increased with N fertilization (P < 0.05, both sites). Belowground NPP, comprising only 6–22% of total corn NPP, was not significantly influenced by N fertilization. When all phases of the crop rotations were evaluated over the long term, OC decay rates increased concomitantly with OC input rates in several systems. Increases in decay rates with N fertilization apparently offset gains in carbon inputs to the soil in such a way that soil C sequestration was virtually nil in 78% of the systems studied, despite up to 48 years of N additions. The quantity of belowground OC inputs was the best predictor of long-term soil C storage. This indicates that, in these systems, in comparison with increased N-fertilizer additions, selection of crops with high belowground NPP is a more effective management practice for increasing soil C sequestration.