EFFECT OF NITROGEN FERTILIZER AND MAIZE STRAW INCORPORATION ON NH4+-15N AND NO3--15N ACCUMULATION IN BLACK SOIL OF NORTHEAST CHINA AMONG THREE CONSECUTIVE CROPPING CYCLES

A pot experiment was conducted to evaluate the effect of nitrogen (N) fertilizer and maize straw incorporation on the accumulation of NH4+-15N and NO3--15N in soil inorganic N pool among three consecutive cropping cycles, aimed to search for an effective N management practice to decrease superfluous accumulation of soil inorganic N and fertilizer N losses. The results showed that the amounts of soil NH4+-15N, NO3--15N and inorganic 15N, and their percent to applied 15N-labeled fertilizer declined significantly with sampling time (p ≤ 0.001). Compared to low N application rate (44.64 mg N kg-1 soil), high N application rate (89.28 mg N kg-1 soil) enhanced significantly the amounts of soil NH4+-15N, NO3--15N and inorganic 15N by 238.6%, 132.9 % and 197.3%, respectively (p ≤ 0.001). In contrast, maize straw addition declined significantly the amounts of soil NH4+-15N and inorganic 15N by 21.4 % and 16.1% compared to without maize straw (p ≤ 0.001). The results suggested that a combined application of chemical fertilizer and maize straw with a wide C/N ratio is an important means for reducing the superfluous accumulation of fertilizer N as soil inorganic N to subsequently lower its loss