Abstract
Freeze–thaw cycles stimulate the release of available soil phosphorus (P) in winter, and biochar as a soil amendment could improve P availability. Nevertheless, it is unclear how freeze–thaw cycles and biochar amendment interact to affect the soil P fractions and their availability in winter, particularly under different soil water conditions. We simulateda freeze–thaw cycle experimentto assess the effects of three factors on soil P fractions: soil moisture content (22%, 31%, and 45%), frequencies of freeze–thaw cycles (0, 1, 3, 6, and 12 times) and biochar amendment (soil and biochar-amended soil). Modified Hedley sequential P fractionation was conducted to measure the soil P fractions. Increasing the number of freeze–thaw cycles increased soil labile P fractions in the soil with the lowest moisture content (22%). After biochar amendment, the content of labile P decreased as the number of freeze–thaw cycles increased. Biochar amendment enhanced P availability in Mollisols owing to the direct effect of NaOH-Po, which has a large direct path coefficient. Principal components analysis showed that moisture content was a major factor influencing the variation in the P fractions. The P fractions were separated by the interactive effects of biochar amendment and freeze–thaw cycles in soils with a higher moisture content (45%), indicating that the effects of freeze–thaw cycles on P availability appear to be more pronounced in biochar-amended Mollisols of higher water contents.
Highlights
As a limiting nutrient for crop growth, phosphorus (P) in soil plays a critical role in plant energy metabolism, biochemical processes, and energy transformation [1]
Significant changes in P fractions induced by freeze–thaw cycles and moisture content are shown in Figure 2 and Table 1, in which the changes of P appear to be more pronounced in soils of higher moisture content (Figure 3)
The increasing number of freeze–thaw cycles could increase the content of labile P fractions
Summary
As a limiting nutrient for crop growth, phosphorus (P) in soil plays a critical role in plant energy metabolism, biochemical processes, and energy transformation [1]. Sui et al [9] modified the method of Hedley et al [6]; they used H2O as the first extractant instead of equilibrating the soil sample with an anion-exchange resin to identify the P fractions in biosolid-amended Mollisols. This method has been widely applied in research on the effects of P fractions in manure-amended soil [10], organic and inorganic P-source-amended soil [11], and fertilized soil [12]
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