This paper reports spatial and temporal changes at the regional level in soil organic carbon (SOC) using a soil-test database. A total of 23 329 SOC test values recorded between 1990 and 2004 by certified commercial laboratories and collected in a mountainous French region (Franche-Comte´) were integrated in a database. Results show a strong trend in organic carbon content, mainly related to elevation. A large loss in SOC was observed over the survey period. This loss correlated with baseline SOC content with greater loss from soils with higher carbon content. This loss is likely to be due to both changes in land use from permanent grassland to cultivation and to an increase in temperature during the survey period. Our study demonstrates that past soil-test results which were not originally intended for monitoring can provide an alternative method for detecting changes in SOC.

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The practice of applying large amounts of P to agricultural land over several previous decades, particularly in regions with intensive livestock production, has resulted in P accumulation in soils with an increased risk of P losses into water bodies and thus of eutrophication. This work is aimed at analyzing the spatiotemporal changes in extractable soil P content at the regional level by means of a soil test database and then comparing results with independent data from agricultural censuses. A total of 228 079 soil P‐test values [Dyer method: 1:5 soil‐citric acid solution (20 g/L) ratio] generated between 1980 and 2003 by certified commercial laboratories in Brittany (northwestern France) were integrated into a database. Changes in P were analysed by considering four 4–6 year periods between 1980 and 2003 using cumulative frequencies and summary statistics performed on raw soil test values. Then data were aggregated into discrete entities (canton: administrative entities) and summary statistics, linear regression and spatial distribution were carried out. P balances were measured for the same entities taken from the agricultural censuses of 1979, 1988 and 2000. Over the entire study period, a marked spatial variability was observed with higher P content in the western part of Brittany, as well as a systematic increase in median P content with lower amplitude over the past decade. The mean cantonal soil P surplus accumulated over 24 years amounted to 763 kg P/ha of usable agricultural land. Similar P balances sometimes gave rise to widely different increases in P. Cumulative balances were positively correlated with an increase in soil P (r2 = 0.34).

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A database including results from 102 000 soil analyses was built for arable topsoil in France. We show a strong effect of soil particle‐size distribution on present organic carbon (OC) stocks in these soils. By using the upper decile values by textural classes as a proxy of the maximal carbon storage potential, we show that it might be more efficient to encourage practices favouring carbon accumulation in soils already having high OC stocks than to try to increase OC stocks in soils where present stocks are low.

Total phosphorus (TP) build-up in agricultural soils represents both a threat to aquatic ecosystems and a valuable resource for future crop production, given the context of increasing food demand combined with the rapid depletion of the world's phosphate reserves. Therefore, it is crucially important (i) to identify the main factors controlling topsoil TP and (ii) to develop methods for mapping its spatial distribution. Multiple linear regression models were used with two distinct approaches to calculate TP and covariates linked to the P cycle. Firstly, covariates were selected from the R eseau de Mesures de la Qualit e des Sols database, the French soil monitoring network, which consists of soil samples collected from 2158 sites on a 16-km regular grid. Secondly, covariates were selected to map TP from spatially exhaustive datasets in France. The first approach explains 80% of variability in topsoil TP. The variables selected are linked to the autochthonous origin of P (parent material), to allochthonous origin (organic carbon and nitrogen contents) and to the retention capacity of soil (Al, Fe, Ca and clay contents). The predicted map obtained from the second approach provides a mean TP of 0.76 g/kg. This study demonstrates that creating national scale maps of TP, based on detailed soil sampling and many variables, is feasible and can be used to model the P cycle and P transfer processes. Such maps can be used in P erosion and transfer models over river basins, and therefore to predict P exports to surface waters.

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