Effects of cover crop systems on soil physical properties and carbon/nitrogen relationships in the coastal plain of southeastern USA

Uncertainty exists concerning the impact of cover crops with conservation tillage on the total agricultural environment. A study conducted from 2002 to 2005 by USDA-ARS and the Univ. of GA assessed the effects of cover crops on soil physical properties and C/N relationships in a sandy coastal plain soil. The cropping systems were (A) sunn hemp (Crotalaria juncea L.), crimson clover (Trifolium incarnatum L.), sweet corn (Zea mays L.); (B) sunn hemp, fallow, sweet corn; (C) fallow, crimson clover, sweet corn; (D) fallow, fallow, sweet corn; or (E) fallow, fallow, fallow. Three N rates (0, 67, or 133 kg ha−1) were tested on the corn for cropping systems A, B, and C, while N rates of 0, 67, 133, 200, and 267 kg ha−1 were used for cropping system D. No N was applied to cropping system E. Soil physical property measurements made on minimally disturbed cores (7.6 cm height × 7.6 cm diameter) collected from the top 7.6 cm both within the row and interrow of each plot seven times during the study included bulk density (BD), saturated hydraulic conductivity (Ks), and volumetric soil moisture content (θ) over a range of matric suctions. Carbon/nitrogen levels were determined from samples of the top 2.5 cm of soil collected quarterly. Three year mean biomass added to the soil from sunn hemp ranged from 6.9 to 9.8 Mg ha−1, while that from crimson clover ranged from 3.3 to 5.0 Mg ha−1. The input of cover crop biomass increased soil C (0.3–4.7 mg g−1) and N (0.1–0.5 mg g−1), which contributed to improvements in soil structure and fertility. Significantly greater C (2.2 mg g−1 vs. 1.8 mg g−1), lower BD (1.71 Mg m−3 vs. 1.73 Mg m−3) and greater θ at field capacity (0.126 vs. 0.113 cm3 cm−3) were found in the rotations with sunn hemp as crop 1 as compared to rotations with fallow as crop 1. There were significant differences in BD, Ks, and θ between soil in the rows and that in the interrows for all treatments. In general, BD were lower, Ks were greater, and θ were greater in the rows than in the interrows. Overall the study indicated that high residue input from fall and winter cover crops is important for adding C, retaining plant-available N in organic matter, increasing fertilizer use efficiency, and improving soil physical properties in the very sandy soils of the southeastern (SE) USA coastal plain region.