Difference in plant availability of nitrogen in aggregates of black soil under long-term fertilization

Objectives The formation mechanism of soil aggregates is different, and the conversion and availability of the contained nitrogen (N) in these aggregates are different as well. The differences in N availability among soil aggregates were investigated in this study to provide theoretical basis for rational fertilization, build-up of good soil structure and the improvement of N fertilizer use efficiency.

Methods An incubation experiment and a pot experiment were conducted in 2014. The soils used were collected from the long-term fertilization experiment of black soil in Jilin province, and involved treatments included: no fertilization (CK), chemical NPK fertilizers (NPK), chemical NPK fertilizers and straw returning (NPKS), and chemical NPK fertilizers and manure (NPKM). Urea with a ¹⁵N abundance of 20.12% was mixed into 1000 g of soil and incubated within 2 L volume of plastic bottles for 40 days at 25℃. The incubated soil was separated into 4 aggregate fractions: coarse free particulate organic matter (> 250 μm, cfPOM), micro-aggregate organic matter (53−250 μm, iPOM), outer mineral-associated organic matter (MOMo, <53 μm), and inner mineral-associated organic matter (MOMi, < 53 μm). Residing differences of fertilizer N in aggregates were analyzed. Ryegrass (Loliumperenne L.) was planted in each soil fraction for 30 days, then the biomass, fertilizer N uptake and fertilizer N use efficiency of ryegrass were analyzed.

Results The plant biomass was the highest in MOMo (100.2 mg/pot) and cfPOM (99.8 mg/pot) in treatments of NPK and NPKM, respectively. The urea N content of ryegrass in three soil fractions was in the order of CK > NPK > NPKS > NPKM. The resided nitrogen in cfPOM was consistent with the nitrogen content of ryegrass, except that it was highest in NPK treatment for the fraction of MOMo. N absorbed by ryegrass was mainly from cfPOM and MOMo (0.1−0.21 mg/pot), and less than 0.05 mg/pot from other fractions. The nitrogen use efficiencies were 14.1%−19.3%, 5.5%−15.4%, 3.1%−4.9%, and 12.7%−23.6% in cfPOM, iPOM, MOMi, and MOMo, respectively. In NPKM treatment, N use efficiency was highest in cfPOM (19.3%), and that in NPK was highest in MOMo (23.6%) .

Conclusions The application of organic fertilizer promotes exogenous N to be stored in large aggregates and held the highest nitrogen efficiency of large aggregates, which is beneficial to the nutrient supply in the later season. The application of chemical fertilizer facilitates the use efficiency of N immobilized by outer mineral-associated organic matter. The residual nitrogen availability is low in inner mineral-associated organic matter though the residual amount is high.