Landscape patterns of catchment and land-use regulate legacy phosphorus releases in subtropical mixed agricultural and woodland catchments.

Landscape composition and configuration determine the exchange of matter and energy among different landscape patches and may affect riverine phosphorus (P) exports derived from watershed legacy sources. However, a lack of understanding of landscape pattern effects on legacy P releases has yielded large uncertainties in mitigating watershed water quality using management practices or landscape planning. This study revealed the significance of legacy effect in the headwater catchments through the time-lag response of the long-term trend of river P exports to the change of net anthropogenic P input (NAPI). By constructing empirical statistical models that incorporated NAPI, hydroclimatic, terrain factors, soil chemical properties, and land use variables, the sources of annual riverine total phosphorus (TP) and dissolved inorganic phosphorus (DIP) exports were divided into current annual NAPI input and legacy sources inputs. The model estimations indicated that the contribution of legacy sources to riverine TP exports was 0.33-1.12 kg ha yr (50.7-82.8%), which was significantly higher than the contribution to DIP exports (0.18-0.49 kg ha yr, 42.4-81.4%) in 2012-2017. Redundancy analysis (RDA) and variance partitioning analysis (VPA) methods were used to quantify the relative contribution of landscape patterns, soil P content, and terrain factors to legacy P releases. Results revealed that the relative contribution of the landscape composition and configuration to the total variations of legacy P releases was greater than that of the soil P and terrain factors. For different land use patches, a large area of woodland with a high aggregation degree and a large area of ponds with multiple net structures may significantly alleviate legacy P releases. In contrast, the legacy P releases were significantly positively associated with highly aggregated agricultural, tea plantation, and residential patches. This study provides theoretical support for strategies aiming to control legacy P from the perspective of landscape planning.