Relationships between soil respiration and photosynthesis-related spectral vegetation indices in two cropland ecosystems

Empirical evidence for the link between vegetation photosynthesis and soil respiration (Rs) is reported for most ecosystems, but the quantification of photosynthesis parameters seldom use remote sensing data. In an attempt to clarify this issue, we focused on the growing season and examined direct relationships between Rs and photosynthesis-related vegetation indices (VIs) in maize and winter wheat agroecosystems. At seasonal time scale, crop biophysical parameters, such as leaf chlorophyll content (Chlleaf) and green leaf area index (GLAI), explained most Rs variation in the maize and winter wheat fields. Among the selected VIs, enhanced vegetation index (EVI) and red edge chlorophyll index (CIred edge) showed stronger correlations with Chlleaf for maize or GLAI for winter wheat than normalized difference vegetation index (NDVI). Moreover, for both fields, the relationship between daily mean Rs and either EVI or CIred edge was consistently stronger than the relationship between daily mean Rs and NDVI. The reason may be attributed to the saturate of NDVI at high vegetation densities and sensitivity of NDVI to background reflectance. Our results demonstrated that simple VIs (i.e. EVI and CIred edge) based entirely on remote-sensing data can provide better correlations with Rs than a number of variables which are more frequently correlated with Rs in field studies for both maize and winter wheat over the growing season. These results will be helpful for the development of future Rs model over a large spatial scale.