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Background: Rice paddy wetlands may serve as a nutrient source or sink in agricultural ecosystems. However, the pattern of nutrient loss on a watershed scale is not clear. A year-round on-site observation study based on 6-h intervals was conducted. Rainfall, temperature, runoff nutrient concentrations, and adjacent stream water samples were automatically monitored to uncover the temporal changes in the runoff losses of the predominant nutrient proxies (total N and total P) from a typical rice-planting area (120 ha) in the Yangtze river delta region of China. Results: A high total N concentration in the rice-planting area was observed during the rice-planting season; however, significant fluctuation in the total P was evident throughout the year. The Δtotal N (drainage minus stream) parameter showed negative values with a mean of −0.25 mg L−1, while Δtotal P showed positive values with a mean of 0.06 mg L−1. The annual average N and P runoff from paddy field were 11.6 kg ha−1 and 1.5 kg ha−1, respectively. The window phases for the total N loss were mainly concentrated in the rice-growing season. However, the window phase for the total P loss was more dispersive throughout the year. No apparent relationships were found between rainfall and N and P concentrations by self-organizing map analysis. Conclusions: The high-resolution monitoring, in this study, suggested that nutrient loss loading rather than nutrient concentration was strongly related to surface runoff Therefore, fertilization before high-intensity rainfall should be avoided to mitigate the nutrient runoff losses and maintain the rice wetland eco-function. [ABSTRACT FROM AUTHOR]

• Colloidal P was closely related to Olsen-P in paddy soils on a regional scale. • Cattle manure biochar amendment decreased colloidal P in paddy soils. • Cattle manure biochar amendment increased available P in paddy soils. Colloidal phosphorus (P coll) and available phosphorus (AP) are of fundamental importance in the nutrient cycle in soil solutions and subsurface drainage of ecosystems. Although many studies have been conducted on soil P loss and P availability, there remains limited focus on the relationship between P coll and AP. In this regional-scale study, soils with loam texture were collected from 63 paddy fields in southern China to explore the relationships between P coll and AP as well as other soil parameters. Subsequently, three typical paddy soils (sandy loam, silt loam, and clay loam) were incubated to study the effect of cattle manure biochar (CMB) amendment on P coll and AP at three application rates (0, 1, and 2%, w/w) for 30 d. The results showed that P coll was closely related to AP (R 2 = 0.453, P < 0.05) and pH (R 2 = 0. 487; P < 0.05) on a regional scale. In addition, clay loam soils contained higher P coll than the other soil types. The CMB amendment increased the AP by 129.0–872.0% but reduced the P coll by 9.65–22.95%, in which colloidal molybdenum-nonreactive phosphorus (MUP coll) was reduced by 7.69–34.15% and colloidal molybdenum-reactive phosphorus (MRP coll) was reduced by 5.13–22.73%. Multiple linear regression revealed that the release potentials of P coll (R2 = 0.735), MRP coll (R2 = 0.543), and MUP coll (R2 = 0.512) could be predicted by the dissolved organic carbon, total carbon, pH, and AP. Overall, this study confirmed that the application of CMB reduced the loss of P coll from paddy soils and enhanced the soil AP content. [ABSTRACT FROM AUTHOR]