Degree of short-term drying before rewetting regulates the bicarbonate-extractable and enzymatically hydrolyzable soil phosphorus fractions

Highlights

• Effects of soil drying intensity on Olsen P and phosphatase-hydrolyzable P_o

• Hydrolyzable P_o represented 34–79% of Olsen P_o that constituted 33–56% of Olsen P_t.

• Olsen P_i and P_o in extremely dry soils increased by 26–48% and 6–53%, respectively.

• Hydrolyzable P_o, mainly monoester P, increased by 11–63% in extremely dried soils.

• Relative changes of Olsen P_i and phytate-like P relied on SOM, Al/Fe oxide contents.

Abstract

Soil drying-rewetting can remarkably affect phosphorus (P) transformation, and thus, alter P distribution among P pools; however, little is known about the effect of the degree of drying before rewetting on labile P fractions and the bioavailability of organic P (P_o). In this study, soils with distinct physico-chemical properties were allowed to desiccate to 5%, 10%, 20%, 30%, and 40% water holding capacity (WHC), and soils maintained at 50% WHC were used as controls, and the bicarbonate-extractable P and hydrolyzable P_o fractions were analyzed after 5 h of rewetting. Bicarbonate-extractable P_o accounted for 33.3–56.4% of extractable total P, and hydrolyzable P_o constituted 34.4–79.7% of extractable P_o. For extractable P_o, 7.7–29.9% was labile monoester P, 6.5–14.9% was diester P, and 17.8–36.5% was phytate-like P. Bicarbonate-extractable inorganic P and P_o contents were not affected by 20–40% WHC treatments, but increased by 26.3–48.1% and 5.7–52.9%, respectively, when the soils were dried to 5% WHC. Similarly, labile monoester, diester, and phytate-like P contents increased by 12.5–89.8%, 0–65.2%, and 24.6–65.6%, respectively, in 5% WHC soils. Pearson's correlation analyses showed that the relative increases in bicarbonate-extractable inorganic P and phytate-like P, following extreme drought, were positively correlated with soil organic carbon, oxalate-extractable aluminum (Al), and iron (Fe), suggesting regulatory roles of organic matter and Al/Fe oxides in P transformation during soil drying-rewetting. Taken together, our results suggest that extreme drought events before rainfall or irrigation facilitate an increase in the level of labile P, including considerable proportions of hydrolyzable P_o fractions, potentially posing a substantial threat to water bodies in the context of climate change.

Introduction

Phosphorus (P) is often a limiting macronutrient in many territorial systems and its transformation is largely driven by fluctuations in soil moisture, such as drying-rewetting (DRW) (Blackwell et al., 2010). Soil DRW, a common form of abiotic perturbation, can remarkably affect microbial immobilization of P (Yevdokimov et al., 2016), alter soil P fractionation (Achat et al., 2012, Butterly et al., 2011, Styles and Coxon, 2006, Turner and Haygarth, 2003), and result in the loss of labile P via leaching and runoff (Blackwell et al., 2013, Pote et al., 1999, Schönbrunner et al., 2012, Turner et al., 2003a). The inorganic P (P_i) fraction, solubilized as a result of DRW alternation, is predominantly available biologically. Soil organic P (P_o) includes, but is not limited to, sugar phosphates, inositol phosphates, nucleic acids, phospholipids, and condensed P; however, its bioavailability is still poorly understood. Numerous studies have suggested that a considerable fraction of dissolved P_o in soils is amenable to hydrolysis, and thus, it might act as a potential source of P for soil microorganisms and plants (Hwang et al., 2015, Jones and Oburger, 2011) and microalgae in water bodies (Sanudo-Wilhelmy, 2006). Therefore, the characterization and determination of soil P_o is of primary importance during DRW alternation.

In fact, numerous studies have quantified P_o pools in soil extracts and leachates, primarily using conventional digestion (Blackwell et al., 2009, Butterly et al., 2011), ³¹P nuclear magnetic resonance spectroscopy (Cade-Menun and Liu, 2014), or high-performance liquid chromatography (Wang et al., 2011). However, such techniques provide little information regarding the availability of the P_o fraction. Alternatively, phosphatase hydrolysis has been used as a novel approach to characterize P_o compounds in various samples (DeLuca et al., 2015, Zhu et al., 2013). Because soil P_o must first be hydrolyzed to P_i for uptake by microorganisms and plants, enzymatic hydrolysis can provide an estimate of hydrolyzable and thus, potentially bioavailable P_o species in soils (Bünemann, 2008). Therefore, enzymatic hydrolysis is preferable for determining soil labile P_o, and more specifically, the hydrolyzable P_o fraction.

In the last decade, few studies reported remarkable changes in the hydrolyzable P_o species when moist soils or animal manures were air-dried (He et al., 2007, Turner et al., 2002a, Turner, 2005). Similarly, other studies that focused on the effects of DRW on P transformation primarily exposed soils to be basically air-dried (Achat et al., 2012, Butterly et al., 2011, Turner et al., 2002a). However, few studies considered that, in semi-arid regions, rainfall distribution is highly uneven among seasons, implying that soils would be dried to different extents before the following rainfall events, or that even within individual sites and areas, the water potential of soils would differ based on the presence and extent of vegetation. Moreover, in nature, soils are rarely dried to extremely low water potentials. Our previous study revealed that labile P pulses in a grassland soil largely depended on the degree of drying during a DRW alternation (Sun et al., 2017). Some studies have suggested that microbes make an important contribution to P pulses following rewetting (Turner and Haygarth, 2001, Turner et al., 2003c); however, our findings suggested that P pulses were primarily of non-microbial origin (Sun et al., 2017). These contrasting findings raise unanswered questions regarding the factors that play a key role in labile P transformation caused by soil DRW and whether changes in levels of labile P are related to soil physico-chemical properties, such as soil organic matter content and soil texture. Accordingly, the goal of this study was to investigate short-term P transformation in soils exposed to various intensities of drying before rewetting and to characterize the P_o fractions by phosphatase hydrolysis. We also evaluated the factors that affected P transformation during soil DRW alternation.