Soil response to surface-applied residues of varying carbon-nitrogen ratios

Abstract

A 5-year study was conducted to monitor patterns of NO3- accumulation following the addition of plant residues of varying C:N ratios to the soil surface, to determine whether the availability of NO3- following these applications would be timely and sufficient for typical annual crop uptake. Microbial respiration was measured in the last 3 years of the study to investigate how microbial activity was related to treatment differences in NO3- accumulation. Treatments included: hairy vetch (Vicia villosa Roth) cut at mid-bloom; wheat (Triticum aestivum L.) straw applied at 4 Mg ha-1; vetch with 4 Mg ha-1 wheat straw; and a bare ground control. Soil NO3- and respiration rates were correspondingly high for the 3–4 weeks following residue placement. Peak NO3--N accumulation in vetch treatments occurred between 25 June and 10 July and ranged from 100 to 168 kg ha-1, with an average of 140 kg NO3−-N ha-1. Nitrate was sufficient and timely enough to meet most summer annual crop needs in the region. Over 5 years, peak NO3−-N was approximately 100 kg ha-1 higher in vetch than non-vetch treatments and roughly twice as high at 0–5 cm than at 5–20 cm. The addition of wheat straw caused a reduction of approximately 20% NO3--N throughout the season as compared to vetch alone.