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Colloidal phosphorus (Pcoll) 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 Pcoll 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 Pcoll 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 Pcoll and AP at three application rates (0, 1, and 2%, w/w) for 30 d. The results showed that Pcoll was closely related to AP (R2 = 0.453, P

We are still challenged to fully understand to what extent controlling irrigation inputs is important for limiting phosphorus (P) losses from paddy soils and how this affects the contribution of dissolved P (DP), particulate P (PP), and colloidal P (Pcoll) fractions in P losses. For this investigation, an internet-based smart irrigation system was developed to precisely implement different irrigation scenarios––flooding irrigation (FI) and two water-saving irrigation (WSI) scenarios of intermittent (II) and controlled irrigation (CI). A simulation experiment was conducted and the concentrations of total P (TP), DP, PP, and Pcoll released to floodwater (FW) and leachate (LW) were determined during a rice season after the addition of different P forms (chemical fertilizer (CP), biogas slurry (BP), and manure (MP)) at a rate of 40 kg P·ha−1. The results revealed that the CI scenario effectively decreased P losses in FW (∼51%) and LW (∼63%) with a positive WSI ratio (39.1%), while II reduced it in FW by ∼11% and in LW by ∼27% with a WSI ratio of 11.0% compared with FI. The BP and MP additions caused a higher risk of P losses than CP under FI, with lower risk under WSI. DP largely contributed to P losses (FW: 30%–71%; LW: 42%–66%); however, more PP was released to FW under II (37%–54% of TP). The Pcoll (FW: 5%–23% and LW: 6%–24%) was more sensitive to release at initial periods after fertilization on account PP. This work provides a promising approach to timely control irrigation inputs, which could limit P losses from paddy soil; however, there remains a need to assess and generalize our initial evidence at field scales.

Agricultural cropping systems involve the application of high fertilizer rates, which lead to phosphorus (P) losses via surface runoff to the aquatic environment, thereby resulting in severe eutrophication. Here, to evaluate the nutrient loss, we established three monitoring stations in Zhejiang Province, China, in each of which the crop types were double cropping rice, rice–wheat rotation, and vegetable cultivation. Field experiments were conducted with four treatments and three replications over a whole planting year. The four treatments at the same P application rate were: 1) no fertilization (control; CK), 2) chemical fertilization (CF), 3) substitution of 30 % chemical P in CF by organic P in solid sheep manure (OF) or in liquid biogas slurry (BS), and 4) substitution of 30 % chemical P in CF by organic P fertilizers (solid sheep manure or liquid biogas slurry) +1.5 t ha−1 polyacrylamide modified BC (PSB). The runoff losses of various P forms, that is, the total P, total dissolved P, particulate P, and colloidal P losses were analyzed. The total P concentration varied greatly with the crop growth stages. Compared with CF treatment, PSB treatment significantly reduced the concentration of P fractions in the runoff in all three cropping systems, while the OF treatment significantly reduced the particulate P and colloidal P concentrations by coating colloidal particles with organic carbon. However, BS treatment increased particulate P and colloidal P concentrations. Compared with CF treatment, PSB treatment reduced total P loss by 41.1 %, 29.7 %, and 37.8 % in the double cropping rice, rice–wheat, and vegetable systems, respectively. The PSB and OF treatments significantly reduced the particulate P and colloidal P losses, while BS treatment increased the loss of various P forms. Compared with the CF treatment, the PSB treatment significantly increased the available P contents by 12.7–85.4 mg kg-1, and decreased the colloidal P contents by 26.7 %–51.4 % in the soils examined. The results of the redundancy analysis confirmed that P loss via runoff was mainly caused by differences in the P inputs and crop types. The results of this study highlight the important role of PSB in reducing P loss via runoff from agricultural cropping systems.

Modified ceramsite has been widely reported as a P adsorbent in wastewater treatments; however, its performance in removing P from agricultural runoff is unclear. A new low-cost ceramsite is modified in this study applying Zn/Fe-layered double hydroxides (LDHs; CZF) to investigate its practicality in treating P-polluted agricultural runoff. The results revealed that the CZF has a theoretical maximum P-adsorption capacity of 11.2 mg-P g−1. Various factors in P-adsorption performance followed the order of pH value (−22.6) > Ca2+ (21.5) > Mg2+ (19.9) > CO32− (−14.5) > SO42− (−13.3) > humic acid (−10.7) > NO3− (−5.57) > Cl− (−2.93) > Na+ (0.32) > NH4+ (0.19). Through iterant adsorption/regeneration, CZF cyclically reduced P concentration in agricultural runoff from 0.58 mg-P L−1 to below 0.1 mg-P L−1 under the preferred parameters of 20 g L−1 dosage, 2 h adsorption time at pH 7.2, and 1 h desorption time at pH 13.0. The treated water has low luminescence toxicity as well as low leaching risks of embedded Zn and adsorbed P in the spent CZF. Additionally, the CZF packing module significantly decreased P in agricultural runoff within four weeks. The CZF dosage and hydraulic retention time (HRT) prolonged the effective operating time (EOT) for P removal, whereas the total organic carbon (TOC) concentration had a negative impact. The cost of CZF for removing P from agricultural runoff was as low as 0.45 ¥ m−3. Overall, this research indicates that using CZF as the adsorption substrate in a ditch purification system is an effective approach for mitigating P pollution in agricultural runoff.