Long-term cropping systems effects on soil aggregate stability, corn grain yields, and yield stability

The growing awareness regarding the ecological and economic impacts of intensive agriculture has elevated interest in developing sustainable cropping systems that provide high crop productivity, reduce reliance on external inputs, and improve soil quality. Two studies which are described in the five chapters of this thesis, assessed long-term cropping systems effects on soil aggregate stability, corn grain yields, and yield stability in a 36-year-old crop rotation and fertility experiment, named the Hunter Rotation Experiment (HRE).;In the first study, we evaluated long-term effects of ten different crops grown in four cropping systems of HRE on soil aggregate stability, estimated by water-stable aggregates (WSA), during four months of a growing season. Water-stable aggregates depend on temporary binding agents such as roots and fungi which are affected by crop management. Because annual plants support shorter live-root periods, we hypothesized that WSA would be higher in soils under perennials (PR) and double-cropped small grains (SG) than in soils under summer annuals in annual systems (SAa), and summer annuals in perennial and diverse systems (SAp). We also hypothesized that WSA would fluctuate more during a growing season in soils under summer annual crops than in soils under PR and SG. Further, the live-root period in a cropping system and soil microbial biomass carbon (SMBC) would predict WSA better than total soil organic carbon and tillage frequency.;We sampled soils of 10 crops from four cropping systems managed under inorganic fertilizers in the HRE: two annual systems: two annual systems: continuous corn (Zea mays L.), corn-soybean [Glycine max (L) Merr.]; a perennial system: 4yr corn-4yr alfalfa ( Medicago sativa L.); and a diverse system: corn-oats (Avena sativa L.)/wheat (Triticum aestivum L.)-2yr red clover (Trifolium pratense L.)+ timothy (Phleum pratense L.) in spring, summer and autumn, 2005. We measured % WSA by slaking and wet-sieving the air-dried 1-2 mm aggregates in water.;The WSA under PR, SG and SAp were respectively 106%, 107%, and 84% higher than under SAa. The WSA under SAp did not differ from the WSA under PR and SG, indicating a more pronounced cropping system effect than individual crop-effect. Water-stable aggregates varied more under the summer annual crops than under PR and SG from spring-autumn. Estimated live-root period predicted WSA best along with soil water content and SMBC (R2= 0.95).;The second study evaluated average corn grain yields, yield trends and yield stability in the four cropping systems under three fertility regimes of the HRE during 1990-2005. We hypothesized that corn yields would be higher, more stable and increase more over time in: (i) perennial and diverse cropping systems compared to annual systems, and (ii) manure-fertility compared to inorganic-fertility regimes. Fertility regimes were inorganic, or manure based on crop N- or P-needs.;Mean yields in 4C4A and COW2RT were 10-12% higher than CC and 7% higher in 4C4A than CS. Yields increases over time did not differ (0.28 Mg ha -1 yr-1) among all treatments. Coefficient of variation (CV) analysis, however, indicated that yield variability was significantly higher in CC (CV = 28%) than in 4C4A (CV = 21%) across fertility regimes.;According to regression stability analysis, response of corn yields to the environmental conditions did not differ among four cropping systems within inorganic and P-based manure fertility. Under N-based manure fertility, however, yields diverged between CC and other systems in the poorest-yielding year but converged in the highest-yielding year. Response of corn yields to the environmental conditions did not differ between manure- and inorganic-fertility, but yields were 7% higher under manure-fertility in the poorest-yielding year.;The results indicate that perennial and diverse cropping systems that rotate corn with perennials and winter double-cropped small grains, can promote higher soil aggregate stability than a corn-soybean or continuous corn system. Compared to CC, perennial and diverse systems are likely to produce higher yields across three fertility regimes, and more stable yields under N-based manure fertility.