Meagan E. Schipanski

Agriculture in 2050: Recalibrating Targets for Sustainable Intensification

http://bioscience.oxfordjournals.org XXXX XXXX / Vol. XX No. X BioScience 1 BioScience XX: 1–6.

Regional Differences in Phosphorus Budgets in Intensive Soybean Agriculture

Fertilizer-intensive agriculture has been integral to increasing food production over the past half century hut has been accompanied by environmental costs. We use case studies of phosphorus fertilizer use in the worlds most productive soybean-growing regions, Iowa (United States), Mato Grosso (Brazil), and Buenos Aires (Argentina), to examine influences of management and soil type on agricultures most prevalent phosphorusrelated environmental consequences: eutrophication and consumption of Earths finite phosphorus reserves. With increasing phosphorus inputs, achieving high yields on tropical soils with high phosphorus-binding capacity is becoming more common. This system has low eutrophication risks but increases demands on phosphorus supplies. In contrast, production in traditional breadbaskets, on soils with lower phosphorus-binding capacities, is being sustained with decreasing phosphorus inputs. However, in these regions, historical overuse of phosphorus may mean continued eutrophication risk even as pressures on phosphorus reserves diminish. We focus here on soybean production but illustrate how achieving sustainable agriculture involves an intricate optimization of local, regional, and global considerations.

Ecologically Based Nutrient Management

The management of nutrients is fundamental to agricultural productivity and viable rural livelihoods. Farmers have the dual goals of supporting crop and animal growth, while minimizing losses to the environment. Nutrient availability is a function of management practices and inherent characteristics of the environment (i.e., climate, soil type), and the organisms in that environment. In this chapter we explore how the application of ecological principles will provide managers with tools for developing coherent nutrient management strategies that optimize the complex processes governing nutrient cycling in agroecosystems. Sustainable management requires close attention to internal nutrient cycling, through building labile/available pools AND nutrient pools that are more easily retained in the soil. Strategic choices are required in orchestrating the full suite of management decisions that contribute to effective nutrient management, including decisions such as which fertility sources to use, and how much to apply in a given field, how to use rotation and intercropping in concert with these sources, which crop genotypes to grow, and how to best use interventions such as tillage or fire. Participatory techniques for assessing current nutrient management practices and developing innovative systems are invaluable in developing ecologically based nutrient management schemes.

Weed Suppression in Cover Crop Monocultures and Mixtures

Interest in planting mixtures of cover crop species has grown in recent years as farmers seek to increase the breadth of ecosystem services cover crops provide. As part of a multidisciplinary project, we quantified the degree to which monocultures and mixtures of cover crops suppress weeds during the fall-to-spring cover crop growing period. Weed-suppressive cover crop stands can limit weed seed rain from summer- and winter-annual species, reducing weed population growth and ultimately weed pressure in future cash crop stands. We established monocultures and mixtures of two legumes (medium red clover and Austrian winter pea), two grasses (cereal rye and oats), and two brassicas (forage radish and canola) in a long fall growing window following winter wheat harvest and in a shorter window following silage corn harvest. In fall of the long window, grass cover crops and mixtures were the most weed suppressive, whereas legume cover crops were the least weed suppressive. All mixtures also effectively suppressed weeds. This was likely primarily due to the presence of fast-growing grass species, which were effective even when they were seeded at only 20% of their monoculture rate. In spring, weed biomass was low in all treatments due to winter kill of summer-annual weeds and low germination of winter annuals. In the short window following silage corn, biomass accumulation by cover crops and weeds in the fall was more than an order of magnitude lower than in the longer window. However, there was substantial weed seed production in the spring in all treatments not containing cereal rye (monoculture or mixture). Our results suggest that cover crop mixtures require only low seeding rates of aggressive grass species to provide weed suppression. This creates an opportunity for other species to deliver additional ecosystem services, though careful species selection may be required to maintain mixture diversity and avoid dominance of winter-hardy cover crop grasses in the spring. Nomenclature: Austrian winter pea, Pisum sativum L.; canola, Brassica napus L.; cereal rye, Secale cereale L., corn, Zea mays L., forage radish, Raphanus sativus L., medium red clover, Trifolium pratense L.; oats, Avena sativa L.; wheat, Triticum aestivum L.

Soil Carbon Pools in Dryland Agroecosystems as Affected by Several Years of Drought

No-till and increased cropping intensity (CI) can increase yield and soil organic C (SOC) in the US Great Plains compared with traditional wheat ( L.)-fallow management. However, gains in SOC and other C pools may not be permanent. Increasing frequency of drought may reduce C inputs and potentially reverse gains accrued during wetter periods. This study examined the effect of drought on the persistence of SOC with two objectives: (i) to determine soil C pools (0-20 cm) after 24 yr in no-till as influenced by potential evapotranspiration (PET), landscape position (slope), and CI; and (ii) to compare the size of the C pools after the first 12 yr (wet) versus the subsequent 12 yr, notable for frequent droughts. Rotations were wheat-corn ( L.)-fallow (WCF), continuous cropping (CC), and a grass Conservation Reserve Program mixture planted across slopes at three sites in Colorado with similar precipitation but increasing PET. After 24 yr, water-soluble organic C increased with CI from WCF to CC to grass with 250, 340, and 440 kg C ha, respectively. Soil microbial biomass C also increased with CI-1500, 1660, and 2135 kg C ha for WCF, CC, and grass, respectively. The particulate organic matter C pool had a three-way interaction with PET, slope, and CI. Overall, between Years 12 and 24, SOC increased in grass by 16.9%, with a rate of 425 kg C ha yr sequestration compared with 10.5 and 1.4% for the WCF and CC systems, respectively.

Climate Change Impacts on Yields and Soil Carbon in Row Crop Dryland Agriculture

Dryland agroecosystems could be a sizable sink for atmospheric carbon (C) due to their spatial extent and level of degradation, providing climate change mitigation. We examined productivity and soil C dynamics under two climate change scenarios (moderate warming, representative concentration pathway [RCP] 4.5; and high warming, RCP 8.5), using long-term experimental data and the DayCent process-based model for three sites with varying climates and soil conditions in the US High Plains. Each site included a no-till cropping intensity gradient introduced in 1985, with treatments ranging from wheat-fallow ( L.) to continuous annual cropping and perennial grass. Simulations were extended to 2100 using data from 16 global circulation models to estimate uncertainty. Simulated yields declined for all crops (up to 50% for wheat), with small changes after 2050 under RCP 4.5 and continued losses to 2100 under RCP 8.5. Of the cropped systems, continuous cropping had the highest average productivity and soil C sequestration rates (78.1 kg C ha yr from 2015 to 2045 under RCP 4.5). Any increase in soil C for cropped rotations was realized by 2050, but grassland treatments increased soil C (up to 69%) through 2100, even under RCP 8.5. Our simulations indicate that reduced frequency of summer fallow can both increase annualized yields and store more soil C. As evapotranspiration is likely to increase, reducing fallow periods without live vegetation from dryland agricultural rotations may enhance the resilience of these systems to climate change while also increasing soil C storage and mitigating carbon dioxide emissions.

The Role of Urban Agriculture in a Secure, Healthy, and Sustainable Food System

Investments in urban agriculture (UA) initiatives have been increasing in the United States, but the costs and benefits to society are poorly understood. Urban agriculture can link socioeconomic and health systems, support education and societal engagement, and contribute to a range of conservation goals, including nutrient recycling and biodiversity conservation. Urban agriculture is spatially dispersed and small scale, creating opportunities to redirect underutilized land, water, and nutrient resources. Urban agriculture reduces water and carbon footprints when it replaces lawns. Labor and time requirements, potential for environmental and nutrient pollution, and scarce water resources are challenges that UA must address. Based on our review of the literature, it is unclear whether UA provides economic or nutritional benefits to urbanites, but our case study shows that UA can provide some benefits when replacing other land uses.

Rhizosphere priming and plant-mediated cover crop decomposition

Background and aimsRhizosphere priming occurs when plant belowground carbon (C) allocation influences the rate of soil organic matter (SOM) decomposition. We investigated the effects of priming and plant-mediated cover crop decomposition on agroecosystem C and nitrogen (N) dynamics.MethodsUsing C stable isotopes, we tracked C and N from corn, clover (Trifolium pratense) and rye (Secale cereale) cover crop litter, and background SOM in plots following clover, rye, or no cover crop (fallow) in 2013 and 2014.ResultsCorn enhanced the decomposition of N-rich clover cover crop litter in 2013, but there was little evidence of priming of bulk SOM decomposition. There was no corn effect on litterbag decomposition in 2014, likely due to greater soil moisture and temperature in no-corn plots. Corn N uptake per unit of corn-derived CO2 respiration was consistently lower following rye than clover and fallow, suggesting a higher C cost for corn to access N following a rye cover crop.ConclusionsThis is one of the first field-based studies to provide evidence that plant-mediated litter decomposition potentially provides an important source of plant-available N. Climate and residue quality influence the extent to which corn mediates its own N supply with implications for agroecosystem C and N cycling.