施肥对麦田土壤可溶性有机氮的影响

利用长期定位试验,研究施肥和小麦生长对土壤可溶性有机氮(EON)的影响。长期不同施肥土壤包括不施肥(No-F)、施用化肥(NPK)和有机肥与化肥配施(MNPK)3种。EON含量范围为7.5—29.3 kg/hm~2,No-F、NPK和MNPK土壤中EON分别占可溶性总氮的40%、56%和56%。长期有机肥与化肥配施显著提高0—15 cm土层EON含量,但对30 cm以下土层EON含量无影响。在小麦开花期,可溶性有机氮的含量及其相对含量显著高于拔节期和收获期。虽然施用氮肥对当季EON含量无显著影响,但同位素示踪微区试验表明,土壤耕层(0—15 cm)中仍有0.4%—2.8%的可溶性有机氮来源于当季施入的肥料氮。可见,化学氮肥向可溶性有机氮的转化缓慢,但农田土壤中可溶性有机氮含量与矿质态氮含量相当,发生淋溶损失的风险大。

Effect of fertilization on extractable organic nitrogen in wheat monoculture cropping systems

Soil extractable organic nitrogen (SON) is an important nutrient pool involved in N transformations, and the content and conversion of SON are affected by fertilization practices. However, many gaps remain in our understanding of SON, especially in agricultural soil. The effects of long-term (1990-2009) fertilization on SON at elongation, flowering, and harvest stages in wheat were evaluated in a loess soil (Eum-Orthic Anthrosol) in northwestern China. The treatments included no fertilization (No-F), application of inorganic NPK fertilizer (NPK), and combined application of inorganic NPK and manure (MNPK). Using ¹⁵N tracer techniques, ¹⁵N-labeled urea (165 kg N/hm²) was applied to microplots within each treatment to investigate the effect of short-term addition of N on content of SON during the wheat-growing season in wheat monoculture cropping systems. The SON content was 7.5-29.3 kg/hm² and accounted for 40%, 56%, and 56% of total extractable N in No-F, NPK, and MNPK, respectively. Compared with No-F, application of inorganic NPK fertilizer increased SON content significantly (55% on average) in the 0-15 cm soil layer. Soil extractable organic N content in the MNPK treatment was significantly higher (by 32%-35%) than that in the NPK treatment in the 0-15 cm layer. Long-term fertilization had no effect on SON content below 30 cm. SON was highest at flowering and was significantly higher during flowering than at the elongation stage in NPK and MNPK (by 48% and 82%, respectively). In relation to No-F, fertilization treatments increased the SON significantly in the 0-100 cm soil profile, SON was 43.1, 51.6, 55.2 kg/hm² in No-F, NPK, and MNPK, respectively. Addition of N had no significant effect on SON content in the 0-15 cm soil layer during the same growing season; however, 0.4%-2.8% of SON was derived from the ¹⁵N-labeled fertilizer applied before seeding, representing 0.03%-0.24% of the fertilizer, and short-term addition of N increased SON in the 0-100 cm soil profile by 35%, 30%, and 14% in No-F, NPK, and MNPK, respectively. We conclude that the conversion of inorganic N to extractable organic N was slow. However, long-term fertilization increased SON content in the topsoil, and SON is a significant nitrogen pool in agriculture soils.