Effects of partial substitution of chemical fertilizer with manure and/or straw on the abundance of functional genes related to soil N-cycling

Objectives This study analyzed the abundance of functional genes related to soil N cycling, potential nitrification rate (PNR) and N₂O emission fluxes under different fertilization patterns to provide a scientific basis for the efficient use of organic materials in greenhouse vegetable production.

Methods We conducted a greenhouse vegetable fertilization experiment on a fixed-site located in Xiqing District, Tianjin City. This experiment started in 2009. There were six treatments with equal NPK rate but different chemical fertilizer (CF) ratio, pig manure (M), and maize straw (S) in the local field experiment. The treatments were 4/4CF, 3/4CF+1/4M, 2/4CF+2/4M, 1/4CF+3/4M, 2/4CF+1/4M+1/4S, and 2/4CF+2/4S. Soil samples were collected at 0–20 cm depth at the full-fruit stage of spring tomato (the 20th vegetable season) to analyse indicators related to soil N cycling.

Results 1) Compared with 4/4CF, the other treatments increased soil PNR by an average of 72.9%; the M-amended treatments increased PNR by 107.0%. The S-amended treatments (P<0.05) decreased soil PNR compared to 2/4CF+2/4M and 1/4CF+3/4M. 2) Compared with 4/4CF, functional genes abundance in organic-amended soils increased by 19.9%, 20.4%, 19.1%, and 2.3% for NapAB, NirS, NorB, and NosZ (denitrification genes), and by 25.9% for NirB (dissimilatory nitrate reduction gene). The abundance of AmoA, AmoB, AmoC, Hao (nitrification genes), and NrfH (dissimilatory nitrate reduction genes) decreased by 37.9%, 46.3%, 33.8%, 65.5%, and 8.8%, respectively. 3) N₂O cumulative emission fluxes during 0 to 28 days increased by 59.6% compared with 4/4CF treatment. 4) The correlation coefficient (r-value) of PNR was 0.37 for soil organic C, 0.47 for ammonium N, 0.56 for N₂O cumulative emission fluxes, 0.78 for NapAB, 0.21 for NirK, 0.53 for NorB, and –0.40 for pH. Moreover, the correlation coefficient (r-value) of soil N₂O cumulative emission fluxes was 0.90 for soil organic C, 0.83 for total N and nitrate N, 0.64 for ammonium N, 0.67 for NapAB, 0.49 for NirK, 0.36 for NirS, 0.88 for NorB, –0.52 for pH, –0.62 for AmoA, –0.64 for AmoB, –0.71 for AmoC, and –0.77 for Hao. The redundancy analysis revealed that soil nitrate-N (P = 0.01), ammonium-N (P = 0.03), and organic C (P = 0.05) had strong effects on soil N cycling functional microorganisms, explaining 34.0%, 13.3%, and 11.3% of the variation in the community structure, respectively.

Conclusions Partial substitution of chemical fertilizer with organic materials, especially replacing 2/4 chemical fertilizer with 1/4 pig manure and 1/4 maize straw, could significantly decrease the abundance of nitrification genes and simultaneously increase denitrification and dissimilatory nitrate reduction genes. This potentially improves tomato N absorption and reduces the amount of N that may be leached downward.