The rice-wheat rotation in southern China is characterized by frequent flooding-draining water regime and heavy nitrogen(N)fertilization. There is a substantial lack of studies into the behavior of dissolved organic nitrogen(DON) in the intensively managed agroecosystem. A 3-year in situ field experiment was conducted to determine DON leaching and its seasonal and yearly variations as affected by fertilization, irrigation and precipitation over 6 consecutive rice/wheat seasons. Under the conventional N practice(300kg N ha-1for rice and 200 kg N ha-1for wheat), the seasonal average DON concentrations in leachate(100 cm soil depth) for the three rice and wheat seasons were 0.6–1.1 and 0.1–2.3 mg N L-1, respectively. The cumulative DON leaching was estimated to be1.1–2.3 kg N ha-1for the rice seasons and 0.01–1.3 kg N ha-1for the wheat seasons, with an annual total of 1.1–3.6 kg N ha-1. In the rice seasons, N fertilizer had little effect(P > 0.05) on DON leaching; precipitation and irrigation imported 3.6–9.1 kg N ha-1of DON, which may thus conceal the fertilization effect on DON. In the wheat seasons, N fertilization had a positive effect(P < 0.01)on DON. Nevertheless, this promotive effect was strongly influenced by variable precipitation, which also carried 1.8–2.9 kg N ha-1of DON into fields. Despite a very small proportion to chemical N applied and large variations driven by water regime, DON leaching is necessarily involved in the integrated field N budget in the rice-wheat rotation due to its relatively greater amount compared to other natural ecosystems.
SONG GeZHAO XuWANG Shen-QiangXING Guang-XiZHU Zhao-Liang
为揭示秸秆源黑炭连续还田对太湖平原稻麦轮作农田土壤生产力和固碳作用的影响,设黑炭施加量为0(CK)、4.5和9.0thm23个处理,通过2 a 4个完整稻麦轮作季的盆栽试验,研究了稻秆来源黑炭每季还田下的稻麦作物产量.养分吸收状况及土壤理化性质的变化.结果显示,土壤w(TOC)(TOC为总有机碳)和w(全N)随黑炭施加量的增加而增加.每季黑炭施加量为9.0thm2时,土壤w(TOC)和w(全N)可分别提高46.7%～113.0%和9.3%～28.3%.黑炭施入土壤后能够提高稻麦作物地上部分生物量,籽粒产量增加11.4%～60.5%,秸秆产量增加15.0%～56.8%.黑炭处理下稻麦作物体内N、P、K、Mg和Ca的累积量显著提高,这一现象与每季结束后土壤w(全N)以及土壤有效元素含量〔w(有效P)、w(有效K)、w(有效Mg)和w(有效Ca)〕的增加相吻合.黑炭施入可显著提高土壤pH和CEC(阳离子交换量),尤其是黑炭施加量为9.0 thm2时,pH最高可达6.79,CEC最高达到12.7 cmolkg.连续三季施入黑炭后,土壤容重比不施黑炭处理降低8.0%～12.2%.试验结果表明,秸秆来源黑炭施入太湖平原稻麦农田可起到固碳增汇、增加土壤碳库容量的作用,也能改善土壤理化性质,提高土壤生产力.
