Identification of soil P fractions that are associated with P loss from surface runoff under various cropping systems and fertilizer rates on sloped farmland

RESEARCH ARTICLE Identification of soil P fractions that are associated with P loss from surface runoff under various cropping systems and fertilizer rates on sloped farmland Xinghua Li1, Baona Wang1, Tewu Yang1,2*, Duanwei Zhu2,3, Zhongnan Nie4, Junchi Xu1 a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 1 MOA Key Laboratory of Crop Ecophysiology and Farming System in the Middle Reaches of the Yangtze River, College of Plant Sciences and Technology, Huazhong Agricultural University, Wuhan, China, 2 The Cooperative Innovation Center for Sustainable Pig Production, Wuhan, China, 3 Laboratory of Plant Nutrition and Ecological Environment Research, Huazhong Agricultural University, Wuhan, China, 4 Department of Economic Development, Jobs, Transport and Resources, Hamilton, Victoria, Australia * Abstract OPEN ACCESS Citation: Li X, Wang B, Yang T, Zhu D, Nie Z, Xu J (2017) Identification of soil P fractions that are associated with P loss from surface runoff under various cropping systems and fertilizer rates on sloped farmland. PLoS ONE 12(6): e0179275. https://doi.org/10.1371/journal.pone.0179275 Editor: P. Pardha-Saradhi, University of Delhi, INDIA Received: October 25, 2016 Accepted: May 27, 2017 Published: June 26, 2017 Copyright: © 2017 Li et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This work was supported by the National Key Research and Development Program of China (grant no. SQ2017YFNC060063) and Research and Demonstration on Agro-Ecological Restoration Technology in Danjiangkou Reservoir Region for the South-to-North Water Transfer Project (grant no. 2007BAD87B09). Soil phosphorus (P) fractions and runoff P concentration were measured to understand the fate of soil P entering surface runoff water during summer cropping season of different double cropping systems under two fertilizer regimes. The dominant form of runoff P was particulate P (PP). Runoff total P (TP) was higher at the vegetative growth stage and lower at the crop reproductive stage. TP and PP were derived mainly from soil Olsen-P, Al-P and Fe-P and amounts increased with sediment content in runoff water. Runoff P discharge was closely related to the changes in soil P forms. Soil Olsen-P, mainly consisting of some Ca2-P and Al-P, was increased by elevating fertilizer rate. Along with crop growth, there were active interconversions among Olsen-P, Org-P, Fe-P and O-Al-P in the soil, and some available P converted into Ca10-P, with O-Fe-P possibly being a transitional form for this conversion. The oilseed rape/corn system had less runoff TP at the early stage, and wheat/ sweet potato system had a lower runoff P at the late stage. Intercropping corn with sweet potato in the field with oilseed rape as a previous crop may be helpful for alleviating runoff P load during the summer in this region. Introduction Phosphorus (P) enrichment has been recognized as the most critical stimulator to water eutrophication [1–3], and agriculture is considered a major source of P contamination in surface waters worldwide [4]. Where P is applied at rates exceeding crop uptake, it may built up in the soil and discharge into waterways through runoff during rainfalls [1, 5]. The identification of soil status that likely generates high runoff P concentrations is critical for developing rational P management strategies in agricultural systems [3]. Attempts have been made using agronomic soil test P (STP) to predict runoff P but the success is limited [6, 7]. They suggested that a PLOS ONE | https://doi.org/10.1371/journal.pone.0179275 June 26, 2017 1 / 16 Soil P fractions that are associated with P loss from surface runoff Competing interests: The authors have declared that no competing interests exist. measure of P buffering (PBI) may complement agronomic soil tests to improve the prediction [8, 9]. Dougherty et al. showed that either DGT (diffuse gradients in thin films) or Colwell P and PBI can be used to predict runoff P from six soils with a diverse range of soil P buffering properties and a wide range of P additions in repacked trays grown with pasture under simulated rainfall [3]. However, P concentration and forms in runoff usually vary considerably depending on soil properties, land use and management, fertilizer application, runoff delivery pathway, and other site-specific factors [9, 10], and the relationships between soil P and runoff P needs further study. Phosphorus in soils exists in many complex chemical forms that differ markedly in their behavior, mobility, and resistance to bioavailability in the soils [11]. Therefore, the fate and transport of soil P vary largely depending on the forms [12]. Soil Olsen-P is considered available to plant [13–15] and therefore widely used to estimate soil P availability [16]; nevertheless, its association with P loss from runoff has not been clarified due to its varying components in different soils. Fractionation of soil P is not only an effective approach for investigating soil P availability and transformation [16, 17], but also can provide useful information for assessing the risk of soil P as the potential sources of eutrophication in aquatic systems [11]. However, it is very hard to identify individual compounds of soil P due to its complex chemistry [18]. The method to divide soil inorganic P (Pi) into various fractions was first developed by Chang and Jackson [13]. Further work on this method has led to a sequential Pi fractionation approach that has been widely used to divide soil Pi into L-P (NH4C1-extractable labile P), Al-P (NH4Fextractable Al phosphates), Fe-P (NaOH-extractable Fe phosphates), O-P (P occluded within oxides extracted with sodium citrate-Na2S2O4 solution) and Ca-P (H2SO4-extractable Ca phosphates) in acid and neutral soils [19]. However, the complex O-P should be subfractionated. In calcareous soils, the majority of Pi exists in various Ca-bound forms with different bioavailability to crops. A procedure was then developed for calcareous soil, which fractionates Ca-P into sub-fractions as Ca2-P [CaHPO4
nH2O], Ca8-P [Ca8H2(PO4)6
nH2O] and Ca10-P [Ca10(PO4)6
(OH)2] based on their availability and solubility [15, 20]. A complete P fractionation should also include organic P (Org-P) in soil. Because Ca-P fraction in noncalcareous soils is rather small [21], Lei et al. proposed a modified scheme, based on previous work, to fractionate soil P as Ca2-P, Al-P, Org-P, Fe-P, O-Al-P, O-Fe-P and Ca10-P [19]. Comparison studies indicated that the new scheme is applicable for acid and neutral soils, with higher P extraction rates than the methods of Chang and Jackson [13] and Gu and Jiang [20]. The response and availability of diff (...truncated)