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The spatial variability of soil organic carbon (SOC) and total nitrogen (STN) levels is important in both global carbon-nitrogen cycle and climate change research. There has been little research on the spatial distribution of SOC and STN at the watershed scale based on geographic information systems (GIS) and geostatistics. Ninety-seven soil samples taken at depths of 0–20 cm were collected during October 2010 and 2011 from the Matiyu small watershed (4.2 km2) of a hilly area in Shandong Province, northern China. The impacts of different land use types, elevation, vegetation coverage and other factors on SOC and STN spatial distributions were examined using GIS and a geostatistical method, regression-kriging. The results show that the concentration variations of SOC and STN in the Matiyu small watershed were moderate variation based on the mean, median, minimum and maximum, and the coefficients of variation (CV). Residual values of SOC and STN had moderate spatial autocorrelations, and the Nugget/Sill were 0.2% and 0.1%, respectively. Distribution maps of regression-kriging revealed that both SOC and STN concentrations in the Matiyu watershed decreased from southeast to northwest. This result was similar to the watershed DEM trend and significantly correlated with land use type, elevation and aspect. SOC and STN predictions with the regression-kriging method were more accurate than those obtained using ordinary kriging. This research indicates that geostatistical characteristics of SOC and STN concentrations in the watershed were closely related to both land-use type and spatial topographic structure and that regression-kriging is suitable for investigating the spatial distributions of SOC and STN in the complex topography of the watershed.

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In the semi-arid region of the Loess Plateau in China, a portable photosynthesis system (Li-6400) and a portable steady porometer (Li-1600) were used to study the quantitative relation between the soil water content (SWC) and trees' physiological parameters including net photosynthesis rate (P n ), carboxylation efficiency (CE), transpiration rate (T r ), water use efficiency of leaf (WUE L ), stomatic conductivity (G s ), stomatal resistance (R s ), intercellular CO 2 (C i ), and stomatal limitation (L s ). These are criteria for grading and evaluating soil water productivity and availability in forests of Black Locust (Robinia pseudoacacia) and Oriental Arborvitae (Platycladus orientalis). The results indicated: To the photosynthesis of Locust and Arborvitae, the SWC of less than 4.5 and 4.0% (relative water content (RWC) 21.5 and 19.0%) belong to ''non-productivity and non-efficiency water''; the SWC of 4.5-10.0% (RWC 21.5-47.5%) and 4.0-8.5% (RWC 19.0-40.5%) belong to ''low productivity and low efficiency water''; the SWC of 10.0-13.5% (RWC 47.5-64.0%) and 8.5-11.0% (RWC 40.5-52.0%) belong to ''middle productivity and high efficiency water''; the SWC of 13.5-17.0% (RWC 64.0-81.0%) and 11.0-16.0% (RWC 52.0-76.0%) belong to ''high productivity and middle efficiency water''; the SWC of 17.0-19.0% (RWC 81.0-90.5%) and 16.0-19.0% (RWC 76.0-90.5%) belong to ''middle productivity and low efficiency water''; the SWC of more than 19.0% (RWC 90.5%) belongs to ''low productivity and low efficiency water''. The SWC of about 13.5 and 11.0% (RWC 64.0 and 52.0%) are called ''high productivity and high efficiency water'', which provides the further evidence for Locust and Arborvitae to get both higher productivity (P n and CE) and the highest WUE L and adaptation to the local environment, respectively.

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