Residue Accumulation and Changes in Soil Organic Matter as Affected by Cropping Intensity in No‐Till Dryland Agroecosystems

Crop residue is a valuable resource in Great Plains dryland agroecosystems because it aids in water conservation and soil erosion control. The objectives of our research were to (i) determine the effect of cropping intensity, climate gradient, and soil depth on levels and changes in soil C, soil N, and residue parameters after 8 yr of no‐till management in dryland cropping systems and (ii) relate soil and residue parameters to soil C and N levels. Surface soil properties and residue parameters were compared in two cropping systems, wheat ( Triticum aestivum L.)–fallow (WF) and wheat–corn ( Zea mays L.) or sorghum [ Sorghum bicolor (L.) Moench]–proso millet ( Panicum miliaceum L.)–fallow (WCMF). The effects were examined on the summit position of a catenary sequence of soils across three environments representing an evapotranspiration (ET) gradient. Soils at the low‐ and medium‐ET sites are classified as Argiustols, and the soil at the high‐ET site is an Ustochrept. There was 3.0 Mg ha −1 of residue in the surface 10 cm of soil compared with 2.7 Mg ha −1 of residue on the soil surface averaged over ET gradient and cropping systems. About 90% of the residue in the soil was found within the 2.5‐cm soil depth. The highest soil organic C (SOC) and soil organic N (SON) were observed in the surface 0‐ to 2.5‐cm depth. There was a trend ( P ≤ 0.16) for the more intense WCMF cropping system to have higher SOC and SON contents than the traditional WF system (C = 6.6 g kg −1 for WF compared with 7.5 g kg −1 for WCMF and N = 0.70 g kg −1 for WF compared with 0.74 g kg −1 for WCMF). From 1985 to 1993, gains in SOC (967 kg ha −1 ) and SON (74 kg ha −1 ) occurred in the surface 0‐ to 2.5‐ and 2.5‐ to 5‐cm depths while losses were observed in the 5‐ to 10‐cm depth (SOC = −694 kg ha −1 ; SON = −44 kg ha −1 ). Climate strongly modified these effects but did not reflect a clear ET gradient effect. The results suggest that higher levels of surface SOC and SON can be attained by increasing cropping intensity under no‐till management.