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Manure is used as a source of nutrients and organic matter for agricultural lands, but its unbalanced use has unfortunately made nitrogen (N) and phosphorous (P) leakage to the environment quite common in Western Europe. One way to optimize the use of manure is variable rate (VR) application, wherein within field spatial variability of soil nutrient provision is accounted for. This work evaluated the economic and environmental benefits of a P-based VR manure application and compared it to an N-based uniform rate (UR) application. An on-line visible and near-infrared spectroscopy sensor was used to collect on-line spectral data from soils in two fields of 7.9 and 5.1 ha with barley and wheat, respectively, in Belgium. A calibration model to predict extractable P (P) in soil was developed by partial least squares regression. Based on the locally predicted P level, the P-fertilizer application rate was calculated. According to P2O5 content in manure, manure application rates were set based on the level of soil P measured with the on-line soil sensor. Based on application maps, a strip experiment was conducted to compare the efficiency of VR and UR treatments. The results showed that P-based VR manure application resulted in almost the same yield as the UR treatment; however, it saved the environment by reducing N by 44 kg/ha and 29 kg/ha, and P2O5 by 18 kg/ha and 12 kg/ha, when applied in the two fields. Since farmers received compensation from the manure providers for allowing the manure to be applied in their fields, the P-based VR manure application using less manure has resulted in a smaller gross margin than the N-based UR method, by which more manure was applied. However, if farmers had to pay for this application (as is the case in some European countries), a P-based VR manure application would result in positive gross margins. It is encouraged to adopt VR manure applications in arable crop production, as both economic and environmental benefits are feasible. [ABSTRACT FROM AUTHOR]

• Soil texture did not affect added OM decomposition in winter. • Less added OM was decomposed with coarser texture during warm and dry summer. • This textural influence was indirect by regulation of soil moisture. • Capillary rise had no pronounced impact on added OM mineralization. • Summer drought significantly preserved added OM in sandy soils. Soil organic carbon (SOC) levels generally increase with increasing clay and silt content under similar climatic conditions because of increased physicochemical protection. Surprisingly though, many silt loam soils in Western Europe have low top SOC levels compared to coarser textured soils. Soil texture also strongly controls soil moisture with consequent indirect impact on heterotrophic activity. Especially during periods of prolonged drought, which seem to be occurring more frequently throughout Europe, it could be expected that this soil textural control of moisture retention decisively affects added organic matter (OM) decomposition. We hypothesized that, contrary to expected textural effects on physicochemical protection of OM, the higher moisture availability in silty soils strongly favors added OM mineralization during periodic drought than sandy soils. Moreover, as a secondary objective, we investigated if during prolonged drought, capillary moisture supply could significantly alleviate the water stress on added OM decomposition in sand with rather shallow groundwater table. Therefore, we established a one-year field trial in which the upper 0.5 m soil layer was replaced by sand, sandy loam and silt loam soil with low SOC. In a fourth treatment, a gravel layer was added beneath the sand layer to exclude capillary rise. Maize residues were mixed homogenously with soil in top 25 cm in all plots. Changes in soil moisture and maize-C mineralization (C maize -min) were measured. We found that soil texture did not affect C maize -min until after about five months and thereafter C maize -min rates were higher in the silt loam than in the sandy soils. Moisture content correlated positively with the C maize -min rate for the sand-textured soils only. Capillary rise did not result in a significant increase in C maize -min in the sandy soil. After one year, a larger share of added C maize was mineralized in the silt loam soil (81 ± 6%) than in the sandy soil (56 ± 7%). These results clearly highlight that soil texture controlled C maize -min indirectly through regulating moisture under the field conditions when the study area faced a period of unusual drought. Moreover, our results imply that, under future climate scenarios, more frequent droughts might lead to a lesser SOC depletion in sandy soils compared to in finer textured soils under similar management. [ABSTRACT FROM AUTHOR]