Land management policy shift influenced seasonal variation of erosion-induced nitrogen and phosphorus outputs from intensive agricultural catchment.

A shift in policy to intensive agricultural production and land management often leads to excessive fertilizer application and accelerated erosion with consequent detrimental effects to water bodies. We investigated the impact of that shift by quantifying the spatial and temporal change in sediment sources and associated total nitrogen (TN) and total phosphorus (TP) pollutants output loads in an intensive agricultural catchment in North China across one year (November 2021-November 2022). We describe the implications of this work for intensive agriculture elsewhere in China and other countries. Seasonal sediment source apportionment was estimated at the catchment outlet using Berillium-7 (7Be) combined with compound-specific stable isotope (CSSI) signatures from sources and sediments. Diagnostic 'fingerprints' in MixSIAR were used to discriminate sediment sources between forest and crop farmland converted from forest (F + C(F)), crop farmland (C), and vegetable farmland (V). Our study identified F + C(F) as the dominant sediment source (mean 55.24 ± 2.91 %), intermediate on V (mean 30.06 ± 2.20 %), and least on C (mean 14.70 ± 2.13 %). Sedimentation ranged from 37.98 ± 3.02 to 89.60 ± 12.68 t·ha-1·event-1 and coincided with shifted land use policy and rainfall distribution. The TN and TP in sediment were both mainly derived from F + C(F) (averaged 22.27 ± 4.26 t·event-1 and 11.62 ± 2.28 t·event-1) and least from V (averaged 1.63 ± 0.29 and 2.09 ± 0.33 t·event-1). Despite being a significant sediment source, V contributed little sediment TN and TP input for eutrophication. Our findings imply that F + C(F) are diffuse sources of catchment pollution over the short term. These results describe the successful use of CSSI and 7Be to cost-effectively quantify the seasonal variation of sediment TN and TP loads from land-use-specific sources in the catchment under shifting land management policy in China with potential for use elsewhere. These findings enable soil conservation strategies and land management practices optimized for implementing targeted pollutant abatement initiatives in intensive agriculture in China and elsewhere.

Vetiver grass hedgerows significantly reduce nitrogen and phosphorus losses from fertilized sloping lands.

Runoff and over-use of fertilizers have been considered as two major factors accelerating the discharge of nitrogen (N) and phosphorus (P) from agricultural fields to surface water. The practice of vetiver grass hedgerows (VGH) can check sediments and runoff pollutants from agricultural fields. However, the efficiency of VGH in reducing N and P losses while maintaining optimum crop yields is still unclear under a recommended fertilization rate. A three-year field experiment was conducted on a 10 o sloping land to know how VGH can reduce the discharge of runoff nutrients to surface water bodies and maintain optimum crop yields, and to understand the relationships between changing soil properties and reduction of sediments N and P due to the adoption of VGH. Five fertilization treatments to VGH were examined under VGH plus organic fertilizer (VGH + OF), VGH plus inorganic fertilizer (VGH + IF), sole organic or inorganic fertilizer (OF or IF) and no VGH and fertilizer (Control). Runoff nutrient pollutants PO4-, NO3--N and NH4+-N were significantly (P < 0.01) reduced by VGH + OF compared to OF by 97%, 94% and 95% and VGH + IF compared to IF by 95%, 88% and 89% respectively for 2012, 2013 and 2014. Sediment nutrients N and P were significantly (P < 0.01) reduced by VGH + OF compared to OF by 98% and 99%, and VGH + IF compared to IF by 94% and 99%, respectively. Improved soil properties by VGH significantly (P < 0.01) reduced runoff pollutants and consequently increased maize yields. Our results imply that runoff erosion, rather than fertilization, is a major driving force for agriculture-derived water pollution. Adoption of VGH with a recommended fertilization rate could significantly reduce N and P nutrient losses from agricultural fields and consequently improve water quality as well as maintaining optimum crop yields on sloping lands.