RÉSUMÉ
Aims: To provide a soil quality assessment frame work and threshold limits for assessing soil quality in Ultisol of subtropical China region. Study Design: Selected minimum data set for soil quality assessment and threshold limits for the study were total carbon, nitrogen, soil pH and phosphorus, biomass carbon, nitrogen and phosphorus, maize grain and fresh potato tuber yields. Soil data (2000-2010), maize grain and fresh potato yield data (2000-2009) from a long term experiment under the Institute of subtropical Agriculture, China were analyzed using the SAS statistical package and means were graphically compared to determine threshold limits for selected data set and fitted into a soil quality model. Place and Duration of Study: The key Laboratory for Agro-ecological Processes in Subtropical Regions, Chinese Academy of Sciences; Institute of Subtropical Agriculture, Changsha, Hunan China long-term experimental site in Taoyuan county, conducted from the year 2000 to 2010. Methodology: Soils samples at the experimental fields were obtained from depths 0-20 cm using an auger at each replicate in triplicates and homogenized to obtain a composite sub sample, air-dried, sieved through 2.0 mm to obtain samples for analysis in the Laboratory. Parameters analyzed for were organic carbon concentration, measured by the combustion method using an automated C/N analyzer (Vario MAX CN, Elemental Co., Germany) while total nitrogen was by the Kjeldahl method of ISSCAS (1978). Microbial carbon, nitrogen and phosphorus levels were determined using the chloroform-fumigationextraction method (Jenkinson and Powlson, 1976; Vance et al., 1987; Brookes et al., 1982) and adopting the conversion factors 0.45 (Wu et al., 1990), 0.45 (Brookes et al., 1985), and 0.29 (Wu et al., 2000) respectively for the C, N and P. Extractable N and Olson P were taken from values obtained from the non fumigated soil samples. Data obtained were statistically analyzed using the SAS package for ANOVA and significant means were separated using the Duncan’s New Multiple Range Test (DNMRT). Treatment means were also matched graphically to delineate critical threshold limits between classes for each parameter. Soil quality was assessed by using the Parr et al. (1992) equation; SQ =ƒ(SP,P,E,H, ER,BD,FQ, MI); where SQ= soil quality, SP= soil properties, P = potential productivity, E=environmental factor, H= health (human/animal), ER= erodibility, BD= biodiversity, FQ= food quality and MI= management input. A score scale of 1 to 5 was used in the assessment of parameters in the model; where 1 is best and 5 is the worst condition. However, E, H, ER, FQ and MI were each scored 1.0 because the long-term experiment has an environmental component, health factor, biodiversity, food quality and management input components that are being optimally managed. Therefore SQ= f(SP, P) was used to assess quality of the Ultisol at the uplands and slope land locations. Results: At the uplands, the practice of maize-rape/marsh residue+NK (8.54gkg-1 C, 1.0 gkg-1 N and 5.67 mgkg-1 P) treatments could be rotated with Maize-rape/nil fertilizer (7.51 gkg-1 C, 0.87 gkg-1 and 0.39 mgkg-1 P) to encourage improved soil quality by allowing for more years with soil carbon sequestration, nitrogen and phosphorus credit than years of depletion and discourage soil degradation. At the slope lands, treatments that combined application of organic and inorganic fertilizer materials [Sweet potato-rape/NP+straw (7.18 gkg-1 C, 0.88 gkg-1 N and 0.38 mgkg-1 P) and Peanut-broadbean/NP+straw (6.81 gkg-1 C, 0.86 gkg-1 N and 0.38 mgkg-1 P)] improved soil quality significantly over time by sequestering significantly higher total carbon, nitrogen and phosphorus better than sole inorganic fertilizer [Sweet potato-rape/NPK (6.52 gkg-1 C, 0.81 gkg-1 N and 0.38 mgkg-1 P)]. Conclusion: Ultisol at the upland positions had better quality (SQ1) than those at the slope (SQ2) positions. Threshold limits for nutrients, pH and yield of maize and Fresh Potato tubers in the subtropical China region Ultisol was developed.