Nutrient and Organic Carbon Losses, Enrichment Rate, and Cost of Water Erosion

ABSTRACT Soil erosion from water causes loss of nutrients and organic carbon, enriches the environment outside the erosion site, and results in costs. The no-tillage system generates increased nutrient and C content in the topsoil and, although it controls erosion, it can produce a more enriched runoff than in the conventional tillage system. This study was conducted in a Humic Cambisol in natural rainfall from 1997 to 2012 to quantify the contents and total losses of nutrients and organic C in soil runoff, and to calculate the enrichment rates and the cost of these losses. The treatments evaluated were: a) soil with a crop, consisting of conventional tillage with one plowing + two harrowings (CT), minimum tillage with one chisel plowing + one harrowing (MT), and no tillage (NT); and b) bare soil: one plowing + two harrowings (BS). In CT, MT, and NT, black oat, soybean, vetch, corn, turnip, and black beans were cultivated. Over the 15 years, 15.5 Mg ha-1 of limestone, 525 kg ha-1 of N (urea), 1,302 kg ha-1 of P2O5 (triple superphosphate), and 1,075 kg ha-1 of K2O (potassium chloride) were used in the soil. The P, K, Ca, Mg, and organic C contents in the soil were determined and also the P, K, Ca, and Mg sediments in the runoff water. From these contents, the total losses, the enrichment rates (ER), and financial losses were calculated. The NT increased the P, K, and organic C contents in the topsoil. The nutrients and organic C content in the runoff from NT was greater than from CT, showing that NT was not a fully conservationist practice for soil. The linear model y = a + bx fit the data within the level of significance (p≤0.01) when the values of P, K, and organic C in the sediments from erosion were related to those values in the soil surface layer. The nutrient and organic C contents were higher in the sediments from erosion than in the soil where the erosion originated, generating values of ER>1 for P, K, and organic C. The value of the total losses of nutrients in the form of triple superphosphate fertilizer, potassium chloride, and urea and limestone by water erosion was higher in CT than in NT. For triple superphosphate, the cost of erosion losses was 29 % higher in NT than in CT, while in urea and limestone, the effectiveness of NT in reducing costs was 65 and 50 %, respectively.

摘要：水蚀会造成土壤养分与有机碳流失，使侵蚀位点周边环境发生富集，并引发经济成本损失。免耕系统可提升表层土壤的养分与碳含量，虽能抑制土壤侵蚀，但其产生的径流富集程度高于传统耕作系统。本研究于1997年至2012年的自然降雨条件下，以腐殖质雏形土（Humic Cambisol）为研究对象，定量分析土壤径流中的养分与有机碳含量及总流失量，并计算此类流失的富集率（enrichment rates，ER）与经济成本。

本研究设置如下处理组：a）作物种植土壤：分别为采用1次翻耕+2次耙压的传统耕作（CT）、采用1次凿式犁耕+1次耙压的少耕（MT），以及免耕（NT）；b）裸土处理：采用1次翻耕+2次耙压（BS）。在CT、MT、NT处理组中，依次种植黑燕麦、大豆、箭舌豌豆、玉米、芜菁与黑豆。

15年试验周期内，共向土壤施加15.5 Mg·ha⁻¹石灰石、525 kg·ha⁻¹氮素（尿素）、1302 kg·ha⁻¹五氧化二磷（重过磷酸钙）与1075 kg·ha⁻¹氧化钾（氯化钾）。本研究测定了土壤中磷（P）、钾（K）、钙（Ca）、镁（Mg）与有机碳的含量，同时检测了径流泥沙中的磷、钾、钙、镁含量。基于上述测定数据，计算了养分与有机碳的总流失量、富集率（ER）及经济损失。

研究结果显示：免耕（NT）提升了表层土壤的磷、钾与有机碳含量；免耕组径流中的养分与有机碳含量高于传统耕作组，表明免耕并非完全意义上的土壤保护性耕作模式。当侵蚀泥沙中的磷、钾与有机碳含量与土壤表层对应值进行相关性分析时，线性模型y = a + bx在显著性水平（p≤0.01）下拟合效果良好。侵蚀泥沙中的养分与有机碳含量高于侵蚀起源地的土壤，使得磷、钾与有机碳的富集率（ER）大于1。以重过磷酸钙、氯化钾、尿素与石灰石计，传统耕作组的养分总流失成本高于免耕组。其中，重过磷酸钙的侵蚀损失成本在免耕组较传统耕作组高29%；而尿素与石灰石的成本降幅方面，免耕组分别实现了65%与50%的成本节约。