Long-Term Cultivation of Fruit Plantations Decreases Mineralization and Nitrification Rates in Calcareous Soil in the Karst Region in Southwestern China

Abstract

Determination of rates of mineralization of organic nitrogen (N) into ammonium-N (NH4+-N) and nitrification of NH4+-N into nitrate-N (NO3−-N) could be used to evaluate inorganic N supply capacity, which, in turn, could guide N fertilizer application practices in crop cultivation systems. However, little information is available on the change of mineralization and nitrification in soils under fruit cultivation systems converted from forestlands in karst regions. In a 15N-tracing study, inorganic N supply capacity in forest soils and three typical fruit crop soils under long-term cultivation was investigated, in addition to factors influencing the supply, in calcareous soils in the karst regions in southwestern China. Long-term fruit crop cultivation decreased soil organic carbon (SOC), total N, and calcium concentrations, cation exchange capacity (CEC), water holding capacity (WHC), pH, and sand content, significantly, but increased clay content. Compared to that of forests, long-term fruit crop cultivation significantly decreased mineralization and nitrification rates to 0.61–1.34 mg N kg−1 d−1 and 1.95–5.07 mg N kg−1 d−1, respectively, from 2.85–6.49 mg N kg−1 d−1 and 8.17–15.5 mg N kg−1 d−1, respectively, but greatly increased the mean residence times of NH4+-N and NO3−-N. The results indicate that long-term fruit crop cultivation could decrease soil inorganic N supply capacity and turnover in karst regions. Both mineralization and nitrification rates were significantly and positively correlated with SOC and total N concentrations, CEC, and WHC, but negatively correlated with clay content, suggesting that decreased soil organic matter and increased clay content were responsible for the decline in mineralization and nitrification rates in soils under long-term cultivation of fruit crops. The results of the present study highlight the importance of rational organic fertilizer application in accelerating soil inorganic N supply and turnover under long-term cultivation of fruit crops in karst regions.

Response of Gross Mineralization and Nitrification Rates to Banana Cultivation Sites Converted from Natural Forest in Subtropical China

“…The gross mineralization and nitrification rates were quantified using 15 N pool dilution techniques [14,35,36]. A series of 30 g samples of fresh soil (oven-dried) were weighted into 250 mL Erlenmeyer flasks for each soil, and these were subsequently pre-incubated at 25 • C for 24 h. After the pre-incubation, 1 mL of 15 NH 4 NO 3 or NH 4 15 NO 3 (10 atom% 15 N excess) solution, containing 1.5 mg NH 4 + -N kg -1 and 1.5 mg NO 3 − -N kg -1 , was evenly applied to the soil in each Erlenmeyer flask. Noticeably, the amount of applied NH 4 NO 3 was relatively higher in this study compared to that previously reported [14,32].…”

“…In soil, inorganic N is produced mainly through the conversion of organic N to NH 4 + (i.e., mineralization) and the subsequent oxidation of NH 4 + to NO 3 -(i.e., nitrification) [11][12][13]. Thus, the determination of gross mineralization and nitrification rates using the 15 N dilution technique can provide a better understanding of the process and intensity of inorganic N production, which has been widely conducted in various ecosystems (e.g., forest, agriculture, grass) [14][15][16]. However, the relevant information regarding changes in gross mineralization and nitrification is limited for banana plantations in tropical or subtropical regions.…”

“…In soil, NH 4 + is lost to the atmosphere through ammonia volatilization, but NO 3 is more easily lost through leaching, runoff, and the emission of nitrogenous gases due to denitrification [28,29], especially in subtropical and tropical regions with high rainfall [30]. This may ultimately lower the sustainable supply capacity of soil inorganic N. Noticeably, this stimulating effect of mineral N fertilizer application on gross mineralization and nitrification rates may gradually decrease with the prolonged cultivation of banana, possibly due to the decline in soil quality [15,31]. For example, long-term rubber or oil cultivation sites converted from forests cause significant reductions in the organic carbon (C) and total N (TN) concentrations in soils, as well as the macro-aggregate levels, but cause increases in micro-aggregate [32,33].…”

“…To verify our hypotheses, soils were sampled from natural forests and banana plantations with different ages of cultivation (3, 7, 10, and 22 y) in the subtropical region of Southwestern China. The purpose of this study was to determine the mineralization and nitrification rates in soils using the 15 N tracing approach, and thus to evaluate the effects of the conversion of natural forests to banana plantations on the soil's inorganic N supply capacity.…”

Effects of Bamboo (Phyllostachys praecox) Cultivation on Soil Nitrogen Fractions and Mineralization

“…Nitrogen (N) is a limiting factor for the growth and development of plants as well as a robust indicator of soil fertility [1,2]. In terrestrial ecosystems, N mainly exists in organic forms in which its availability is restricted by its chemical composition and structure [3].…”