Incorporation of fallow weed increases phosphorus availability in a farmer’s organic rice fields on allophanic Andosol in eastern Japan

We investigated the amount of soil phosphorus (P) in a farmer’s paddy fields under organic farming (OF) for various periods from 0 to 22 years as well as other farmers’ fields under conventional farming. All the fields are located in allophanic Andosol with long history of P fertilizer application, and some of them have been converted to OF across years. After conversion to OF, P was supplied only with winter fallow weeds mainly Foxtail (<i>Alopecurus aequalis</i>), rice residues (rice bran and straw), and guano. We determined total-P (Tot-P) and plant-available P (Av-P), which consists of Truog-P (Tru-P) and Bray-2-P under reducing condition with ascorbic acid (Asc-P), in soils of each field. For both Av-Ps, the ratio to Tot-P increased across years under OF following quadratic functions with both linear and quadratic terms being statistically significant. The ratios showed little changes for the initial 15 (Tru-P) or 10 (Asc-P) years and increased rapidly thereafter. These temporal changes in Av-P were consistent with the rapid increase of the amount of P accumulated in the winter fallow weeds and incorporated in the fields after beginning of OF. These results led us to the hypothesis that the incorporation of winter weeds has contributed to the increase of Av-P in the organic fields across years. We tested this hypothesis by investigating temporal changes of Av-P after suspending the weed incorporation for 2 consecutive years in plots of the organic fields. Both Av-Ps were significantly greater in plots with continued weed incorporation (CWI) than those in plots with its suspension. We further found that the increase of Asc-P in plots with CWI was 4.9-fold the input of total P in the incorporated weeds. This suggests that the incorporation of winter fallow weeds enhanced soil-P availability beyond the supply of P accumulated in the weeds.

本研究针对0至22年不同耕作年限的有机农业（organic farming, OF）稻田，以及常规耕作稻田的土壤磷素（P）含量开展了调查。所有供试田块均分布于长期施用磷肥的水铝英石型暗色土（allophanic Andosol）中，其中部分田块经多年转换已采用有机耕作模式。转换为有机耕作后，田块磷素补给仅依赖冬闲杂草（主要为看麦娘（Alopecurus aequalis））、水稻残留物（稻糠与稻草）以及鸟粪。本研究测定了各田块的土壤全磷（total-P, Tot-P）与植物有效磷（plant-available P, Av-P）；其中植物有效磷包含Truog法提取磷（Truog-P, Tru-P），以及抗坏血酸还原条件下的Bray-2法提取磷（Asc-P）。两类有效磷占全磷的比例在有机耕作下均随耕作年限呈二次函数增长，且一次项与二次项均达统计学显著水平。该比例在初始15年（Tru-P）或10年（Asc-P）内变化平缓，此后快速升高。植物有效磷的这一时间变化趋势，与有机耕作启动后冬闲杂草累积并还田的磷素储量快速增长的情况一致。基于上述结果，我们提出假说：冬闲杂草还田是有机稻田土壤有效磷随耕作年限升高的核心驱动因素。为验证该假说，我们在有机稻田样地中设置连续2年暂停杂草还田的处理，调查此时土壤有效磷的时间变化特征。两类有效磷含量在持续杂草还田（continued weed incorporation, CWI）样地中均显著高于暂停杂草还田的样地。进一步研究发现，持续杂草还田样地中Asc-P的增长幅度是还田杂草总磷输入量的4.9倍，这表明冬闲杂草还田对土壤磷素有效性的提升作用，远超杂草自身累积磷素所能提供的补给。