Estimating Accumulated Biomass and Nitrogen Pools in Over-Wintered Cover Crops: Digital Image Analysis Versus Manual Sampling.

Over-wintered cover crops reduce risk of soil erosion and inorganic nitrogen (N) losses and raise soil organic matter levels over time. A simple method is needed to predict total dry matter (DM) and N credits from cover crops for the following crop. The aim of this study was to determine DM and N pools in over-wintered cover crop stands by (1) manual harvesting, and (2) digital image analysis. Representative samples of cereal rye, wheat and oats (n=94) were identified in farmers' fields using a rectangular quadrat (0.199 m²) to delineate the sampling area. A digital image (5-Megapixel resolution) was taken of each quadrat area. Cover crop maximum and canopy height were recorded. Weeds were separated from cover crop shoots. Roots (washed to remove soil) and shoots were dried separately at 60ºC to stable weight to determine DM and analyzed for total carbon (C) and N. Digital images were processed to measure ground cover (green pixels/total pixels) in each quadrat. Total cover crop DM ranged from 0.007 to 4.3 Mg/ha, representing N pools from 0.3 to 134.2 kg/ha. The C:N ratio was variable (12.7:1 with SD ± 4.1:1). Above-ground DM was strongly correlated with total DM (P<0.0001, r²=0.99, RMSE=0.124 Mg/ha) and N pools (P<0.0001, r²=0.90, RMSE=9.26 kg/ha), suggesting that above-ground biomass can be used to accurately estimate total DM and N pools. Ground cover was positively correlated with total DM (with weeds) (P<0.0001, r²=0.68, RMSE=0.589 Mg/ha) and N pools (P<0.0001, r²=0.66, RMSE=17.1 kg/ha). The relationship improved in a model predicting square root transformed N pools, with canopy height, past year inorganic N fertilization rate, and their interaction as additional covariates (P<0.0001, r²=0.90). A validation set was gathered in spring 2013 to further evaluate potential to accurately estimate N credits from cover crops.