Mixed application of biochar, maize straw, and nitrogen can improve organic carbon fractions and available nutrients of a sandy soil

Biochar with maize straw and nitrogen application may provide better crop growth conditions with respect to soil C/N, organic carbon fractions, and available nutrients than biochar applied alone. An incubation experiment was conducted in which biochar of maize straw pyrolyzed at 500 °C was applied with maize straw and nitrogen over 150 days. The treatments consisted of three rates of biochar applied at 0 (BC), 30 (BC3), and 60 g kg−1 (BC6), two rates (0 and 2 g kg−1) of maize straw (S), and two rates (0 and 0.1 g kg−1) of nitrogen (N) fertilizer (ammonium bicarbonate). The control (CK) was blank soil unfertilized with BC, N, and S. Sampling was done at 30, 60, and 150 days, respectively. The treatments of BC3 and BC6 significantly increased the content of soil total organic carbon by 840–1735%, oxidizable organic carbon (KMnO4–C) by 151–250%, dissolved organic carbon (DOC) by 221–976%, microbial biomass carbon (MBC) by 113–274%, available phosphorus (AP) by 969–4820%, and available potassium (AK) by 715–2413% in the sandy soil. Biochar reduced the ammonium nitrogen and nitrate nitrogen (NH4+–N and NO3−–N) contents, while it increased the carbon-nitrogen ratio (C/N) to 42. Nitrogen addition increased the contents of NH4+–N by 395% and NO3−–N by 661% compared with the CK after 30 days. Straw only affected the contents of DOC and MBC. The S + N treatment increased the content of MBC and available nitrogen compared with the CK. The contents of KMnO4–C, DOC, MBC, NH4+–N, NO3−–N, AP, and AK in the BC + N and BC + S treatments were higher than those in the BC treatments. The BC + N and BC + S treatments could decrease the high C/N that resulted from biochar alone. The contents of KMnO4–C, DOC, MBC, NH4+–N, NO3−–N, AP, and AK in the BC + S + N treatment were higher than those in the BC + S and BC + N treatments. Therefore, we recommend using BC6 + S + N to improve the organic carbon fractions and available nutrients in sandy soil. If straw is lacking in the field, BC6 + N is a suitable alternative.