Optimization of manure recycling and fertilizer use to meet crop nutrient demands and reduce nutrient losses, a case study in Quzhou, China

Summary

CONTEXT: The average manure recycling ratio in China is lower than 40 %, implying that over half of the nutrients in manure is lost to the environment. To reduce environmental pollution caused by nitrogen (N) and phosphorus (P) losses, it is crucial to enhance manure recycling and reduce N and P fertilizer inputs. Manure provides nutrients and reduces soil acidification due to high levels of potassium (K), calcium (Ca) and magnesium (Mg). OBJECTIVE: The study aims to optimize regional manure recycling in view of crop nutrient demands and environmental impacts, using soil nutrient balances accounting for soil P fertility, N losses to air and water and soil acidification. METHODS: We identified optimal manure recycling ratios by calculating required crop nutrient demands while minimizing soil N and P surpluses and soil acidification. The approach was applied to Quzhou, a typical Chinese city consisting of six counties. RESULTS AND CONCLUSIONS: Results show that current manure inputs satisfy almost all crop nutrient demands in Quzhou city except for N and K. Full manure recycling provides sufficient crop nutrients except for N. Combining full manure recycling with an attainable increase in N use efficiency (NUE) and an associated decline in N inputs reduced N losses by 46 %, of which 36 % is due to the NUE increase and 10 % due to enhanced manure recycling. Due to the relatively high soil P status, however, the calculated long-term potential for manure recycling based on the build-up and maintenance approach, called optimal recycling, was only ca 50 %. In the short term, this percentage equaled 68 % and 100 % when the total dose to bring all soils to optimum P levels was spread over 20 or 10 years, respectively. Differences in optimal manure recycling ratios among the counties depended on the availability of local manure resources, cultivated crop areas and soil P status. Consequently, optimal recycling ratios were higher than current ratios in four counties and lower in two counties. Although a higher regional manure recycling rate generally reduced N losses and counteracted soil acidification, these ratios were maximized in view of the soil P status, underscoring the need for maximum manure production levels. SIGNIFICANCE: This study shows that enhancing manure recycling is limited when soils have a high P status, thereby reducing the required P input to avoid potential adverse impacts on water quality. The environmental space for P should be considered in policies promoting regional manure recycling.