Search

Understanding controls on soil organic carbon (SOC) will be crucial to managing soils for climate change mitigation and food security. Climate exerts an overarching influence on SOC, affecting both carbon (C) inputs to soil and soil physicochemical properties participating in C retention. To test our hypothesis that climate, C inputs, and soil properties would differently affect particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), we sampled 16 agricultural sites (n = 124 plots) in the United States, ranging in climate (mean annual precipitation (MAP)—potential evapotranspiration (PET; MAP-PET)), soil pH (5.8–7.9), and soil texture (silt + clay = 13–96%). As MAP-PET increased, soils increased in oxalate-extractable iron (FeO) and aluminum (AlO), decreased in exchangeable calcium (Caex) and magnesium (Mgex), and received greater C inputs. Soil physicochemical properties did not strongly predict POC, confirming the relative independence of this SOC fraction from the soil matrix. In contrast, MAOC was well predicted by combining AlO + [1/2]FeO with Caex + Mgex in a 'matrix capacity index', which performed better than individual soil physicochemical properties across all pH levels (r > 0.79). Structural equation modeling indicated a similar total effect of MAP-PET on MAOC and POC, which was mediated by total C inputs and the matrix capacity index for MAOC but not POC. Our results emphasize the need to separately conceptualize controls on MAOC and POC and justify the use of a unified soil matrix capacity index for predicting soil MAOC storage. [ABSTRACT FROM AUTHOR]

Anecdotal accounts regarding reduced US cropping system diversity have raised concerns about negative impacts of increasingly homogeneous cropping systems. However, formal analyses to document such changes are lacking. Using US Agriculture Census data, which are collected every five years, we quantified crop species diversity from 1978 to 2012, for the contiguous US on a county level basis. We used Shannon diversity indices expressed as effective number of crop species (ENCS) to quantify crop diversity. We then evaluated changes in county-level crop diversity both nationally and for each of the eight Farm Resource Regions developed by the National Agriculture Statistics Service. During the 34 years we considered in our analyses, both national and regional ENCS changed. Nationally, crop diversity was lower in 2012 than in 1978. However, our analyses also revealed interesting trends between and within different Resource Regions. Overall, the Heartland Resource Region had the lowest crop diversity whereas the Fruitful Rim and Northern Crescent had the highest. In contrast to the other Resource Regions, the Mississippi Portal had significantly higher crop diversity in 2012 than in 1978. Also, within regions there were differences between counties in crop diversity. Spatial autocorrelation revealed clustering of low and high ENCS and this trend became stronger over time. These results show that, nationally counties have been clustering into areas of either low diversity or high diversity. Moreover, a significant trend of more counties shifting to lower rather than to higher crop diversity was detected. The clustering and shifting demonstrates a trend toward crop diversity loss and attendant homogenization of agricultural production systems, which could have far-reaching consequences for provision of ecosystem system services associated with agricultural systems as well as food system sustainability. [ABSTRACT FROM AUTHOR]

The potential for mitigating increasing atmospheric carbon dioxide concentrations through the use of terres-trial biological carbon (C) sequestration is substantial. Here, we estimate the amount of C being sequestered by natural processes at global, North American, and national US scales. We present and quantify, where possible, the potential for deliberate human actions - through forestry, agriculture, and use of biomass-based fuels - to augment these natural sinks. Carbon sequestration may potentially be achieved through some of these activities but at the expense of substantial changes in land-use management. Some practices (eg reduced tillage, improved silviculture, woody bioenergy crops) are already being implemented because of their economic benefits and associated ecosystem services. Given their cumulative greenhouse-gas impacts, other strategies (eg the use of biochar and cellulosic bioenergy crops) require further evaluation to determine whether widespread implemen-tation is warranted. INSET: In a nutshell:. [ABSTRACT FROM AUTHOR]

The use of commercial nitrogen (N) fertilizers has led to enormous increases in US agricultural productivity. However, N losses from agricultural systems have resulted in numerous deleterious environmental impacts, including a continuing increase in atmospheric nitrous oxide (N20), a greenhouse gas (GHG) and an important catalyst of stratospheric ozone depletion. Although associated with about 7% of total US GHG emissions, agri-cultural systems account for 75% of total US N20 emissions. Increased productivity in the crop and livestock sectors during the past 30 to 70 years has resulted in decreased N20 emissions per unit of production, but N20 emissions from US agriculture continue to increase at a rate of approximately 0.46 teragrams of carbon dioxide equivalents per year (2002-2009). This rate is lower than that during the late 20th century. Improvements in agricultural productivity alone may be insufficient to lead to reduced emissions; implementing strategies specifically targeted at reducing N20 emissions may therefore be necessary. INSET: In a nutshell:. [ABSTRACT FROM AUTHOR]

The article discusses research which examined possible impact of agricultural management practices which include irrigation, tillage and nitrogen (N) fertilization on soil greenhouse gas (GHG) emissions in the U.S. It is said that irrigation can increase both carbon dioxide and N dioxide emissions unlike strategies which include increasing soil water availability and microbial activity. It reports experimental procedures conducted which include measurement of soil GHG fluxes and application of N fertilizers. The use of the Analysis of Repeated Measures technique for evaluating soil temperature and water content is also noted.

Phenolic plant secondary metabolites actively participate in a broad range of important reactions that affect livestock, plants and soil. In soil, phenolic compounds can affect nutrient dynamics and mobility of metals but their role in northern Great Plains agroecosystems is largely unknown. We evaluated the effects of three phenolic compounds on plant nutrient extractability in agricultural soil. Soil samples (0–10 cm) from pasture and cropped sites near Mandan, North Dakota, USA were treated with water (control) or aqueous solutions of increasingly complex compounds; benzoic acid (BA), gallic acid (GA), or β -1,2,3,4,6-penta- O -galloyl- d -glucose (PGG) at four concentrations (1.25, 2.5, 5 or 10 mg compound gram −1 soil). We measured extractable nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and manganese (Mn) in treatment supernatants and after a subsequent incubation in hot water (16 h, 80 °C). While significant quantities of nitrogen were extracted with water, each compound reduced N extracted in a concentration dependent manner with greatest response to BA (about 25%). However, PGG reduced the solubility of N only during the hot water incubation, suggesting its effects are mostly on organic forms of N. Unlike N, GA and PGG increased extraction of P, relative to water, while BA had less effect. Extraction of the major cations, K, Ca, and Mg, was strongly increased by BA and GA but unaffected by PGG. Extraction of Mn was increased most by treatments of GA but less so by BA. The effects of PGG on Mn were consistent with GA when expressed on a molar basis. These findings suggest plant secondary compounds affect nutrient dynamics in soil, and thus may be part of future management strategies to improve nutrient-use efficiency. [ABSTRACT FROM AUTHOR]

The article reports on the necessity of management systems to improve soil quality and regenerate ecological processes. It is indicated that conserving and restoring soil can supply soil productivity and sustain environmental quality. The Agricultural Research Service (ARS) of the U.S. Department of Agriculture created the Greenhouse Gas Reduction through Agricultural Carbon Enhancement network (GRACEnet) to study soil C sequestration and greenhouse gas emissions.

• We developed water budgets for 18 Long-Term Agroecosystem Research sites. • We estimated uncertainty of all the water budget components. • The average water budget uncertainty was 25% of precipitation. • Uncertainties were greatest for percolation and evapotranspiration. • LTAR sites span a 0.4 – 1.1 evaporative fraction and a 0.8 – 4.8 aridity index. Understanding the movement and storage of water within agricultural landscapes as functions of management and climate is essential for more efficient and sustainable water use. However, knowledge of water storage and fluxes on U.S. agricultural lands is largely incomplete. The Long-Term Agroecosystem Research (LTAR) network provides a unique and geographically diverse set of agricultural study sites in the United States. The objectives of this study were to: 1) characterize the hydrologic variability across the LTAR network; 2) identify data gaps in the water budgets across the LTAR network; and 3) identify opportunities to leverage the LTAR network to improve understanding of water budgets across agricultural landscapes. For each of the 18 LTAR sites, we developed water budgets on an average annual basis. Uncertainty propagation methods combined individual component uncertainties to calculate an overall water budget uncertainty. Datasets length ranged from three to 50 years. The network covers a range of precipitation from 240 to 1400 mm yr−1, evapotranspiration from 228 to 1080 mm yr−1, and surface runoff and subsurface flow from negligible to 560 mm yr−1. However, uncertainties of where all the water is going remained high, in part because soil water storage and downward movement of water were often neglected or measured for very short periods, resulting in average water budget uncertainty of 25% of the water inputs. More accurate measurement of the major inputs and outputs, and direct measurement of water content and percolation are key to understanding how agricultural lands affect terrestrial water budgets. [ABSTRACT FROM AUTHOR]