Nutrient cycling and balance in farmland ecosystem in Bohai Lowland Plain

It is important to optimize nutrient management and improve soil fertility and fertilizer use efficiency for sustainable development of agriculture. This can help to understand nutrient input/output and balance in farmland and the state of soil fertility and its change. Based on national economic statistics for Nanpi County for 1985, 2000 and 2014, the state of nutrient cycle and balance in agro-ecosystems was analyzed for the period 1985–2014. The study also explored the state and change characteristics of soil nutrients by comparing data on nutrients in the topsoil in 1981 (when the second national soil survey was conducted) with that in 2015. The results indicated a significant change in nutrient input/output and balance for 1985 through 2014. There was a significant increase in NPK input. Total inputs of N, P and K increased from 10 701 t in 1985 to 23 386 t in 2014, which represented an annual increase of 2.33%. The sources of N, P and K were different, with N and P mainly coming from chemical fertilizers, followed by organic fertilizers such as manure and crop straw. However, K was mainly from organic fertilizers. Nutrient absorption by crops was the main component of nutrient output, accounting for 80% of total nutrient output. NPK output increased from 18 846 t in 1985 to 90 093 t in 2014, with an annual increase of 2.17%. Taking into account apparent nutrient balance, there were huge N and P budget surpluses since 1985. For the period 1985–2014, P surplus (26.9%–65.5%) exceeded N surplus (16.8%–34.2%). Based on the availability of organic N, available N budgets were respectively 18.1%, 6.5% and 7.8% in 1985, 2000 and 2014, which shifted from surplus to deficit conditions. K balance changed from deficit to surplus condition, improving from a deficit of 33.5% in 1985 to a surplus of 33.6% in 2014. Due to the effect of nutrient balance, the contents of soil organic matter, total N and available P significantly changed, respectively increasing from 8.62 g·kg^-1, 0.542 g·kg^-1 and 2.0 mg·kg^-1 in 1981 to 14.0 g·kg^-1, 0.908 g·kg^-1 and 20.8 mg·kg^-1 in 2015. The total increases in 2015 over those in 1981 were 62.4%, 67.5% and 9.4 times, respectively. The changes of available N and available K were not very noticeable, increasing from 70.5 mg·kg^-1 and 141 mg·kg^-1 in 1985 to 71.8 mg·kg^-1 and 147 mg·kg^-1 in 2015, representing total increases of 1.8% and 4.2%, respectively. The measures of increasing soil fertility and fertilizer use efficiency included scientific and rational fertilization, combined application of organic fertilizer and inorganic fertilizer, straw return to soils and improved fertilization methods. Under the current soil fertility and crop planting structure, nutrient management countermeasures was to optimize N dose, to control application of P and to increase application rate of K to limit nutrient surplus in the environment.