Understanding the impact of long-term partial substitution of chemical nitrogen fertilizer with organic fertilizers (partial organic fertilizers substitution) on soil nitrogen components, mineralization, and availability is necessary to foresee nitrogen (N) dynamics. The present study was conducted a long-term field experiment to investigate the effect of 12 years of fertilizer application on soil nutrient concentrations, enzymatic activities, and N mineralization in a rubber plantation. Treatments included: unfertilized as control (CK), 100% recommended dose of chemical fertilizer (CF), and integrated application of 50% chemical and 50% organic fertilizer (cow manure) (CF+M). The soil physicochemical properties, including total nitrogen (TN) and six labile N components (microbial biomass nitrogen (MBN), particulate organic nitrogen (PON), dissolved organic nitrogen (DON), light fraction organic nitrogen (LFON)), ammonium nitrogen (NH 4 +-N), and nitrate nitrogen (NO 3 --N); five soil enzymes (urease (UE), leucine aminopeptidase (LAP), N-acetyl-glucosaminidase (NAG), Acid phosphatase activity (AcP), and β-1,4-Glucosidase (BG)), and soil organic nitrogen mineralization were determined. Compared with CF, CF+M treatment significantly increased soil pH, TN, MBN, LFON, DON, PON, NH 4 +-N, NO 3 --N, organic carbon (SOC), total phosphorus (TP), available phosphorus (AP), and available potassium (AK) concentrations, while significantly decreased the soil bulk density (BD) and the proportion of soil silt and clay particles. In 0-20 cm soil layer, CF+M treatment significantly decreased the activities of BG but increased AcP. Meanwhile, CF+M treatment significantly increased the NAG and LAP activities in the topsoil layer and UE activities in the subsoil layer. CF+M treatment had high cumulative mineral N production (N t) and N mineralization potential (N 0) but were low net soil N ammonification rates (Net N AM), net soil N nitrification rates (Net N NM), and net soil N mineralization rates (Net N Min) than CF. The piecewiseSEM analysis showed that 99% of the variation in N t and 97% of N Min were explained, with TN and it’s labile components and soil physicochemical properties being the most important direct influencing factor for N t and Net N Min, respectively. Conclusively, partial organic fertilizers substitution could facilitate N availability and soil N supply capacity by affecting soil organic N mineralization and improving soil environmental condition of the rubber plantation. These results suggest that the combine application of chemical fertilizer and manure is a useful management practice and provide theoretical guidance and scientific basis for rational fertilization of rubber plantations in the tropics.

Studies on land degradation and development (LDD) and the underlying factors can help land restoration. In this study, the LDD of Hainan Island from 2000 to 2018 were measured by change vector analysis using the normalised difference vegetation index and net primary productivity (NPP). The underlying factors of LDD were selected from seven potential factors (nature factors: mean annual temperature, mean annual drought intensity, human disturbances: gross domestic product (GDP), population, population urbanisation, accumulated afforestation area, construction area) based on structural equation modelling. The results showed that 1) the degree of land-use changes into water area and construction land were higher than for the other land-use types; 2) Compared to 2000, the most serious degradation occurred in 2015, which accounting for 68.98% of the total land degradation as well as more than three-quarters of the land degradation in all surveyed years was driven by NPP; 3) The proportion of land degradation (PLD) was negatively correlated with the mean value of the land degradation and development (MLDD) in the period of five years, 2000-2018 (P<0.01); 4) The key influencing factors of PLD were temperature, accumulated afforestation area, population urbanisation, in 2000–2005, 2005–2010 and 2010–2018, respectively; 5) The key influencing factors of MLDD were temperature, GDP, population urbanisation, in 2000–2005, in 2005–2010 and in 2010–2018, respectively. Therefore, to minimise land degradation it is necessary to limit the speed of population urbanisation occurs and to improve the island environment’s adaptability to extreme climates.