Phosphorus (P) loss from agricultural areas to waterbodies is a worldwide concern. However, the effect of soil source and transport factors, as clay (C) content and slope (S) degree, under the magnitude of the P transport in Brazilian Subtropical soils still incipiently know. The objectives of this study were i) to quantify the loss of P fractions by runoff in areas receiving pig slurry application and with variations in S and C content; ii) propose an environmental critical limit model of P (P-threshold) for Brazilian Subtropical soils. Thus, two series of experiments were conducted from 2016 to 2018, one under a Nitisol with 642 g kg ^-1 of C and another under a Cambisol with 225 g kg ^-1 of C. The treatments were four P rates (0, 56, 112 and 224 kg ha ^-1 year ^-1) superficially applied as pig slurry, on Tifton (Cynodon sp) pasture, and three S (10, 20 and 30% in the Nitisol and 15, 25 and 35% in Cambisol). P losses increased in both soils as the S and P rates rase. The clayey soil showed P losses three times higher than the less clayey one. Soil S above 25% promotes P losses at a rate three times higher than in soil below this limit. Consequently the P-threshold is proposed in Brazilian Subtropical soils being “P-threshold = (42.287 + C) - (0.230 S + 0.0123 CS)” in soils with a S of up to 25% and " P-threshold = (42.287 + C) - (-0.437 S + 0.039 CS) ” in soils with a S of more than 25%, where both C and S are shown in percentage. The soil clay content and slope degree are aggravating factors to the P transfer process, thus must be considered in suitable models to predict the P losses risk.

从农业地区到水体的磷（P）损失是全世界关注的问题。然而，在巴西亚热带土壤中，在P运移的幅度下，土壤来源和运移因子（如粘土（C）含量和坡度（S）度）的影响仍然是人们所了解的。这项研究的目的是：i）定量计算在施用猪粪便的地区中径流中P组分的损失，以及S和C含量的变化；ii）提出巴西亚热带土壤的环境临界极限模型P（P阈值）。因此，从2016年到2018年进行了两个系列的实验，一个实验是在Nitisol下使用642 g kg ^-1的C，另一个实验是在Cambisol下使用225 g kg ^-1的C.处理为四种P值（0、56 ， 112和224 kg公顷^-1- ^1年）表面用作猪粪便，在提夫顿（Cynodon sp。）和三个S（尼蒂索尔占10％，20％和30％，坎比索尔占15％，25％和35％）。随着S和P比率的提高，两种土壤中的P损失均增加。黏性土壤的磷损失比低黏性土壤高3倍。高于25％的土壤S促进磷的损失，其速率比低于此限值的土壤高三倍。因此，在巴西亚热带土壤中建议P阈值为“ P阈值=（42.287 + C）-（0.230 S + 0.0123 CS）”，在S高达25％的土壤中，“ P阈值=（42.287 + C）-（-0.437 S + 0.039 CS）”在S大于25％的土壤中，其中C和S均以百分比表示。土壤黏土含量和坡度是P迁移过程的恶化因素，因此必须在合适的模型中考虑预测P损失的风险。