GPP and maximum light use efficiency estimates using different approaches over a rotating biodiesel crop

Abstract

This paper presents: (a) results of gross primary production (GPP) 8-d estimated values using a light use efficiency model (LUE) in a non-irrigated rotating rapeseed crop in the upper Spanish plateau, and (b) inter-comparison results of observed GPP with those concurrently retrieved by MODIS. The rotation scheme over the four-year study comprised rapeseed, wheat, peas and rye. Rapeseed, peas and, in part, rye grew under well-watered conditions whereas wheat was dominated by drought.Input data for the LUE model were the fraction of PAR absorbed (FPAR) 8-d products supplied by MODIS (FPARMODIS), in situ photosynthetic active radiation (PAR) measurements and a scalar f varying between 0 and 1, to take into account the reduction of the maximum PAR conversion efficiency (ɛ0LUE) under limiting environmental conditions. In this study, f values were assumed to be dependent on air temperature (T) and the evaporative fraction which was considered a proxy of water availability. ɛ0LUE, a key parameter in LUE models, which varied according to land use, was derived through the results of a linear regression fit between observed GPP and concurrent GAPAR estimates defined as the product of PAR, FPARMODIS and f. Overall, the LUE model provided satisfactory results, R2=86.3%, significantly improving GPP MODIS estimates (GPPMODIS), R2=71.8%. GPPMODIS uncertainties have primarily been attributed to differences in the f stress factor involved in its formulation (fMODIS) depending on vapour pressure deficit and T which did not fully describe the environmental stress conditions at the measuring site.Overall, ɛ0LUE yielded 3.33±0.10gCMJ−1 although this varied depending on crop architecture, phenology and prevailing meteorological conditions. Crop-to-crop ɛ0LUE ranged from 2.74±0.17 to 3.95±0.19gCMJ−1 for peas and rye, respectively, yielding intermediate values for rapeseed and wheat, 2.92±0.18 and 2.86±0.23gCMJ−1, respectively. ɛ0MODIS, derived from the linear fit of GPP versus GPPMODIS estimates, yielded 2.13±0.10gCMJ−1 and crop-to-crop ranged from 1.28±0.17 to 2.41±0.12gCMJ−1 for wheat and rapeseed, respectively. The best linear fits corresponded to crops growing under well-watered conditions, rapeseed and peas, and the worst fits were for wheat, affected by drought. GPP annuals were 1680, 710, 730 and 1410gCm−2 for rapeseed, wheat, peas and rye, respectively.