Soil erosion due to land use change is a major concern in agricultural areas. There is an urgent need to develop appropriate agricultural practices that promote higher biodiversity and improve soil properties. Little is known about the processes of natural vegetation recovery after disturbance by human activities in tropical humid climates. This is particularly the case for dispersion by water (hydrochory) that affects seeds. The displacement of native plant seeds on the soil surface by runoff may be influenced by soil properties and by the use of biochar in agriculture. As these effects have not been documented in tropical agroecosystems and on steep slopes, we present here an in-field experiment on a steeply sloping maize field affected by rainfall and tillage erosion in Northern Thailand. Our objectives were to test the effect of catena position and biochar use on seed displacement, soil loss and nutrient losses. We used 24 plots of 1 m² located at four positions in the catena. Two treatments were tested: (1) conventional system with tillage and (2) biochar incorporated into the soil. We measured the displacement of plastic beads (to imitate round seeds), runoff volume, soil and nutrient losses and soil surface features during in-situ rainfall simulations. After two simulated rainfall events and four natural rainfall events, total bead exportation was 7.2%. We found that catena position and biochar had an impact on runoff, soil and nutrient losses. In this study, previously incorporated biochar reappeared at the soil surface after 8 months under natural conditions and acted like a crust at the soil surface. This biochar crust increased runoff, increased potassium concentration and decreased phosphorus concentration in runoff, decreased bead displacement and had no effect on soil loss. This study, under tropical climate and steep slope conditions, highlights differences in soil surface features and runoff along the catena. We should consider catena position for improving soil management and using appropriate agroecological practices.

中文翻译：

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坡位和生物炭影响热带农业生态系统中的土壤性质和种子置换

土地利用变化引起的水土流失是农业地区的一个主要问题。迫切需要发展适当的农业实践，以促进更高的生物多样性和改善土壤特性。热带潮湿气候下人类活动干扰后自然植被恢复的过程知之甚少。对于影响种子的水（水生植物）分散情况尤其如此。径流对土壤表面的原生植物种子的置换可能受到土壤特性和农业中使用生物炭的影响。由于这些影响尚未在热带农业生态系统和陡坡上记录，因此我们在此展示了一项在泰国北部受降雨和耕作侵蚀影响的陡坡玉米田的现场实验。我们的目标是测试链位置和生物炭使用对种子置换、土壤流失和养分流失的影响。我们使用了 24 个 1 m 的地块²位于链中的四个位置。测试了两种处理方法：（1）常规耕作系统和（2）生物炭掺入土壤。我们在现场降雨模拟过程中测量了塑料珠的位移（模拟圆形种子）、径流量、土壤和养分流失以及土壤表面特征。经过两次模拟降雨事件和四次自然降雨事件后，珠粒出口总量为 7.2%。我们发现链状位置和生物炭对径流、土壤和养分流失有影响。在这项研究中，先前掺入的生物炭在自然条件下 8 个月后重新出现在土壤表面，并在土壤表面起到结皮的作用。这种生物炭结皮增加了径流，增加了径流中的钾浓度和降低了磷浓度，减少珠粒位移，对土壤流失没有影响。这项研究在热带气候和陡坡条件下，突出了沿链的土壤表面特征和径流的差异。我们应该考虑改善土壤管理和使用适当的农业生态实践的连锁地位。