Abstract An improved understanding of potential soluble phosphorus (P) loss in run‐off and leachate from agriculturally managed soils presents practical and theoretical challenges. Our study aimed to discover whether modified Morgan extractable P ( MMP ) can be used to predict water‐soluble P ( WSP ). We first addressed the relationship between MMP and WSP , and whether MMP is useful for predicting the WSP concentrations demanded by water quality regulations. Secondly, we applied novel soil chemical models to explain why the relationship between MMP and WSP depends upon soil properties. Thirdly, we explain how soil properties relate to potential soluble P loss in situations in which soil is subjected to a wide liquid‐to‐soil ratio (e.g. run‐off and rivers) compared with those with a narrow ratio (e.g. soil porewater). To address these P loss scenarios, 60 agricultural topsoils (0–10 cm) were collected from a mixed‐farming catchment (Lunan catchment, northeast Scotland) and chemically characterized. Theoretical understanding of P solubility was obtained with a P sorption model. The data showed variability in the relationship between MMP and WSP . Modelling shows the MMP versus WSP relationship is nonlinear, depending on several confounding factors (P sorption capacity ( PSC ), Ca, pH ) and the liquid‐to‐soil ratio (L:S) employed for WSP determination. Consequently, the slope of the relationship is not unique but depends subjectively on the set of soils surveyed. MMP versus WSP at large L:S (e.g. in run‐off or rivers) is positively correlated to PSC , whereas at narrow L:S (e.g. porewater) there is a negative correlation with PSC . The study provides new ideas for the interpretation and extrapolation of agronomic soil test data for soils of varied properties and highlights the need to utilize insights from soil chemistry.
Agriculture needs to reduce inputs of inorganic fertilizers and close the loop on nutrients that can otherwise become environmental pollutants. This can be achieved by promoting recycling of nutrients within the agricultural landscape. We investigated the extent to which plants found in riparian buffer zones have the potential to provide nutrients to crops as a green manure, through plant growth and decomposition studies. Under controlled conditions, species typical of Scottish riparian buffer strips were tested for their ability to accumulate biomass and nutrients in tissue under N- and P-replete conditions and whether this ability enhanced the utility of the resulting green manure in promoting crop growth. In this proof-of-concept study, we found that green manure derived from riparian buffer strips did not effectively replace inorganic fertilizer and only had a significant positive effect on growth, yield, and nutrient accumulation in barley ( L.) when it was integrated with the addition of inorganic fertilizers. The individual species tested varied in the amount of P they accumulated in their tissue (1.38-52.73 mg P plant), but individual species did not differ in their ability to promote yield when used as a green manure. Our results indicate that selecting certain species in the buffer strip on the basis of their nutrient accumulating abilities is not an effective way to increase the utility of buffer strip green manure as a nutrient source for crops.
Advances in quantifying the spatial variability of soil properties made for agricultural soils are not being mirrored for naturally structured upland soils. The objectives of this study were to determine the degree of spatial variability and variance structure of cation exchange chemistry in a granitic, heather moorland site (Northeast Scotland). Two 20 by 20 m soil plots, a Typic Placaquod and a Typic Humaquept, were sampled at O and B horizon depths at 104 locations in a regular grid overlayed with cluster (Placaquod) and transects (Humaquept) patterns. Soils were analyzed for pH and exchangeable cation, physical, and hydraulic properties. Results showed strongly significant vertical differences in exchange chemistry between surface organic and mineral horizons at either site for all chemical properties. Strong lateral variability was also apparent within the plots. Coefficients of variation (CV) were 18 to 52% for O horizon chemical properties, with similar variability between sites. In B horizons CV values were 26 to 119%, the highest associated with Humaquept chemical properties. Compared with the Placaquod the Humaquept had lower mean pH, but higher mean concentrations of exchangeable Ca and Mg in both horizons. Geostatistical analyses highlighted a generally strong degree of spatial dependence to Placaquod properties, particularly in the organic horizon where correlation ranges were greater. By contrast, the majority of properties for the Humaquept showed random, pure nugget variance indicating no spatial correlations at this scale of observation. It is postulated that seasonal water logging of the Humaquept may explain some differences in the exchange chemistry between sites. These differences in chemistry and in the spatial patterns of variability have implications not only for modeling the role of such soils in controlling the hydrochemical environment of the uplands, but also for the design of field soil sampling strategies for accurately quantifying soil properties.
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