Residing differences of fertilizer nitrogen in aggregates of black soil under long-term fertilization treatments

Objectives Stabilities of organic nitrogen in different aggregates are different. The response mechanism of soil nitrogen fixation to a long-term different fertilization was investigated by analyzing organic nitrogen distribution in different stable aggregates.

Methods The investigation was based on the long-term experiment in black soil. Soil samples were collected from four treatments: no fertilization (CK), chemical fertilization (NPK), chemical fertilization and straw (NPKS), and chemical fertilization and manure (NPKM). The soil samples were incubated for 40 days and the ¹⁵N-labeled urea was applied in the same amount as in the long-term field experiment. Wet sieve method was used to separate aggregates into coarse free particulate organic matter (> 250 μm, cfPOM), micro-aggregate organic matter (53–250 μm), and mineral-associated organic matter (< 53 μm, MOM). The micro-aggregate organic matter was then subdivided into fine free particulate organic matter (ffPOM) and physically protected intra-aggregate particulate organic matter (iPOM), while the mineral-associated organic matter was subdivided into inter- and external- aggregate MOM. The composition of the tested aggregates and their mean weight diameter (MWD) were investigated, and the proportion of the fixed nitrogen in these aggregates was analyzed.

Results The amounts of ¹⁵N-labeled nitrogen fixed by soil in the NPKM and NPKS treatments were significantly increased by about 10% than those in the CK and NPK. The mean weight diameter (MWD) in the NPKS and NPKM (0.34 mm) was significantly higher than those in the NPK and CK (0.23–0.25 mm). Compared with the CK and NPK treatments, the weight percentage of cfPOM was significantly increased by about 10%, while the weight percentage of MOM was 6.0%–10% lower in the NPKM and NPKS treatments. The nitrogen fertilizer resided in cfPOM was in the range of 18–36 mg/kg, and the nitrogen fertilizer resided in MOM was in the range of 18–30 mg/kg.

Conclusions Combined application of chemical fertilizers with manure or straw promoted the accumulation of fertilizer nitrogen in large aggregates, and reduced the accumulation in the clay particles. This might be one of mechanisms for the enhanced nitrogen availability and absorption by manure application.