Crop residue : a hero's journey from biomass to soil carbon in eastern Colorado dryland crop rotation systems

Abstract

Crop residues play a vital role in reducing the potential for wind erosion of agricultural soils in arid and semi-arid regions. The residues act via three modes: reducing wind speed, acting as a physical impediment to wind reaching the soil surface, and as an organic matter input to spur aggregation and aggregate stability. The interactions of crop residues, crop rotation systems, and wind erosion factors were studied at three long-term agricultural research sites along an evapotranspiration gradient near Sterling, Stratton, and Walsh, Colorado. The sites have a 30-year history of dryland, no-till management, and are divided into different cropping system intensities that vary in the frequency of summer fallow periods in the rotation. Crop rotations studied here include wheat (Triticum aestivum)-fallow, wheat-corn (Zea mays) – fallow, and continuously cropped plots with small grains and forage crops including foxtail millet (Setaria varidis) and forage sorghum (Sorghum bicolor). Forage crop and wheat residues were tracked over two growing seasons (2015 and 2016) to estimate the length of time before soil surface cover fell below a 30% threshold and to create models for residue persistence. Decomposition Days (DD), a calculation that factors in temperature and rainfall to estimate cumulative conditions that favor decomposition, was used to normalize time scales following harvest across sites and years. Wheat residue covered 82% of the soil surface following harvest and summer forage crops covered 56%. Wheat persisted longer, taking 62.5 DD to fall to the 30% cover threshold, forage crop residue remained above the threshold for 16.6 DD. The decline of forage crop residue cover followed an exponential decay model. Wheat residue surface cover had a longer, slower decline and fit a quadratic decay model. Wheat stem heights were taller following harvest and heights declined at a similar or faster rate than forage crops. To assess rotation legacy impacts on soil erodibility, soils were sampled in May 2015 and tested for dry aggregate size distribution, dry aggregate stability, and carbon distribution by size classes and between cropping intensities. No differences were found in the amount of erodible aggregate size fraction (<0.84mm) by cropping system intensity. The site with the highest amount of clay in the soil displayed a significant difference in aggregate stability by crop rotation, with wheat-fallow rotations having stability of 2.96 ln J/Kg and continuously cropped systems having 2.80 ln J/Kg. Carbon distribution did not differ by crop rotation but did differ by size class at the site with the highest potential evapotranspiration and lowest clay content where the largest aggregates contained the highest proportion of carbon. Every phase (i.e., rotation year) of each of the crop rotation systems were represented each year. There was a significant difference in mean erodible fraction and aggregate stability by cropping phase at the time of sampling at the site with the highest clay content. Taken together, the crop residue and soil aggregate portions of the study indicate that the reliable and consistent prevention of wind erosion by crop system intensity may be more dependent upon annual crop residue surface cover than longer-term management impacts on soil aggregation properties. The differences in aggregate stability by crop type could be due to the impacts of active root systems at the time of sampling. More investigation is warranted into the influence of active root systems on macro dry aggregates and whether dry aggregate stability properties differ by season. Further study into the application of residue biomass decay models to residue soil cover, particularly in crops with multiple layers of residue is also indicated.