|Mitchell, Jeff - U OF CA, PARLIER, CA|
Submitted to: Soil and Water Conservation Society
Publication Type: Book / Chapter
Publication Acceptance Date: August 24, 2010
Publication Date: December 10, 2010
Citation: Dabney, S.M., Meisinger, J.J., Schomberg, H.H., Liebig, M.A., Kaspar, T.C., Delgado, J.A., Mitchell, J., Reeves, D.W. 2010. Using cover crops and cropping systems for nitrogen management. p. 230-281. In Delgado, J.A. and R.F. Follett (eds) Advances in Nitrogen Management for Water Quality, Ankeny, IA. Interpretive Summary: Cover crops can have multiple benefits on N management whether occupying the land for six weeks or six months: they can reduce N leaching losses, reduce erosion N losses, fix N, immobilize N, and increase crop N uptake. Knowledge and proper management are the keys to maximizing benefits. Cover crops increase the input and cycling of C, N and other nutrients in agricultural systems. Long term accumulation of soil organic C and N improves the physical and biological properties of soil, which can increase the productivity and the water and nutrient use efficiencies of crops. The most sustainable agricultural systems will be achieved by maximizing long-term cover crop nutrient and C inputs and retention without creating short-term water and nutrient deficits. Optimal cover crop management varies by region and cropping system. Benefits are frequently maximized by adopting conservation tillage systems that retains cover crop residues on the soil surface to improve water infiltration. To capture N and reduce environmental losses, cover crops used as catch crops should have rapid root growth potential and should be planted as early as possible. In many situations, to increase N availability to subsequent crops in the form of green manure, cover crops should be killed in the winter or early spring to avoid preemptive competition and speed release of accumulated N. Because legumes’ N fixation is facultative, mixtures of legumes and non-legumes offer the flexibility to either improve N scavenging when soil N is abundant, or increase growth and N accumulation when soil N is limited. Cover crops can increase yields and quality of crops. Summer cover crops with limited irrigation can be used as water management tools to increase water use efficiencies. However, managers must properly control weeds and diseases, maximize nutrient cycling and availability, and control water usage to ensure that no yield reduction or decrease in crop quality occurs. Cover crops can be used viably across the US, and we recommend that those considering using cover crops investigate local practices to determine cover crops’ potential to maximize yields, crop quality and environmental conservation.
Technical Abstract: The reasons for using cover crops and optimized cropping sequences to manage nitrogen (N) are to maximize economic returns, improve soil quality and productivity, and minimize losses of N that might adversely impact environmental quality. Cover crops and cropping systems’ effects on N management are inseparably linked to their influences on soil water balance and the soil carbon (C) cycle. Cover crops decrease N leaching by: (1) decreasing water percolation by increasing evapotranspiration (ET), (2) removing N from soil water through root uptake, or (3) increasing rooting depth or productivity of subsequent crops. Rotating crops with different water use patterns or requirements may also increase total N uptake. Legume cover crops or rotation crops can contribute N to cropping systems through symbiotic N fixation. Soils with dynamic nutrient cycling may result in improved N use efficiency through improved synchrony of N supply with crop demand. Cover crops increase available soil water when increased infiltration exceeds ET. On the other hand, excessive water use by cover crops can deplete soil water and adversely impact yields of subsequent crops if potential ET exceeds precipitation. In semi-arid climates, the benefits of cover crops may be optimized not by maximizing cover crop residue production or cover crop N fixation, but rather by producing an optimal amount of cover crop biomass to maintain soil structure and stimulate nutrient cycling to maximize the growth, productivity, and N uptake of subsequent crops. The optimal selection of cover crop species or mixtures (legumes, cereals, brassicas), growth duration, and cropping sequence need to be site and cropping system specific. Computer models such as NLEAP GIS enable the analysis of high risk cropping system/landscape combinations to achieve optimal N management.