Effects of biochar in combination with cattle slurry and mineral nitrogen on crop yield and nitrogen use efficiency in a three-year field experiment

Abstract

Applying biochar can improve soil characteristics and functions, resulting in significant increases in yield in tropical soils. However, whether biochar can boost soil yield in temperate climates with high soil fertility is mostly unknown. The optimal biochar application rates and combinations with organic and mineral nitrogen (N) fertilizers remain to be determined. To address this research gap, a factorial three-year field experiment in southern Germany was conducted using mineral N fertilizer (0, 60, 120, and 180 kg ha−1 a−1), and biochar (0, 500, and 1000 kg ha−1 a−1 biochar C) mixed with cattle slurry before application (0 and 100 kg ha−1 a−1 slurry N). The effects on crop (silage maize, winter wheat, and winter barley) yield, N uptake, and N use efficiency were studied. Spectral measurements were taken at selected growth stages, and the vegetation index REIP (Red-Edge Inflection Point) was calculated.The silage maize dry matter yield reached 27.2 t ha−1 while the winter wheat and winter barley grain yield reached 10.4 t ha−1, and 9.6 t ha−1, respectively. Biochar consistently, but mostly non-significantly, decreased crop yield (e.g., winter wheat, 5.09, 4.74, and 4.50 t ha−1) and N uptake (74.3, 70.8, and 67.4 kg ha−1) at biochar C rates of 0, 500, and 1000 kg ha−1 a−1 when biochar was mixed with cattle slurry before field application in treatments without mineral N input in all trial years. In the third trial year, the biochar-related yield reductions were significant. Consequently, N use efficiency decreased slightly but significantly. At high additional mineral N fertilizer rates no yield-reducing effects were observed. Biochar applied alone (without cattle slurry or mineral N fertilizer) did not affect crop yield. Positive yield effects of biochar were not observed in any of the trial years.The vegetation index REIP decreased non-significantly with increasing biochar addition to cattle slurry in all three years and correlated very closely (up to r = 0.97) with crop yield.The results show that the yield effect of biochar depends on the combination with organic and mineral fertilizers. The derivation of optimal ratios of biochar, cattle slurry and mineral fertilizer requires further investigation. Based on the yield effects, however, it can be deduced that the more biochar is used, the higher the compensation with nitrogen fertilizer must be to maintain the yield level.