Carbon and Nitrogen Content of Winter Cover Crop Dry Matter.

Cover crop carbon and nitrogen content determines the rate of cover crop decomposition. Carbon to nitrogen (C:N) ratio of plant dry matter is determined mainly by species and age. Grasses have higher C:N ratios than legumes, but C:N ratios increase for all plants as they age and enter reproductive growth stages. Generally, cover crops with a carbon to nitrogen ratio (C:N) < 20 decompose quickly, and release N that can be taken up by primary crops. Cover crops with C:N ratios >30 decompose slowly, immobilizing N and sometimes lowering the soil N supply which in turn can inhibit growth and yield potential of a subsequent corn crop. Thus, knowing the C:N ratio and total amount of N of cover crop biomass can help inform decisions on whether additional N will be needed for corn following cover crops.

We tested winter cover crops planted September 3, 2015 (early planting) and October 14, 2015 (late planting) and sampled April 15, 2016 at Mead, NE. Cover crop treatments were rye, legumes (hairy vetch and winter pea and a mix of rye, hairy vetch, winter pea, plus 4 other species. Cover crops were dried and dry matter was analyzed for C and N using combustion analysis.

Our hypotheses were i) C:N ratio is higher for rye than legumes and intermediate for the mix; ii) C:N ratio is higher for cover crops planted earlier; iii) total cover crop N is highest for rye.

We found that C:N ratio and total N differed with species and planting date. Rye had the highest C:N ratio at 26:1, but N immobilization at this ratio is not likely. Legumes had the highest percentage of N in their dry matter, but produced very little biomass. Rye produced the most biomass which contained up to 35 kg/ha of N.