Alum split applications strengthened phosphorus fixation and phosphate sorption in high legacy phosphorus calcareous soil

Highlights

• Single or split alum applications increased p fixation in calcareous soil.

• Organic matter weakened the p stabilisation of single alum application.

• Alum split applications induced the formation of the non-crystalline fe/al.

• Alum split applications transformed more soil labile p to non-labile p.

Abstract

High phosphorus (P) saturation arising from historic P inputs to protected vegetable fields (PVFs) drives high P mobilisation to waterbodies. Amendment of soils with alum has shown potential in terms of fixing labile P and protecting water quality. The present 15 month pot experiment investigated P stabilisation across single alum application (Alum-1 treatment, 20 g alum/kg soil incorporated into soil before the maize was sown), alum split applications (Alum-4 treatment, 5 g alum/kg soil incorporated into soil before each crop was sown i.e. 4 × 5 g/kg) and soil only treatment (Control). Results showed that the Alum-1 treatment caused the strongest stabilisation of soil labile P after maize plant removal, whereas the P stabilisation effect was gradually weakened due to the transformation of soil non-labile P to labile P and the reduced active Al³⁺ in soil solution. For the Alum-4 treatment, soil labile P decreased gradually with each crop planting and was lower than the Alum-1 treatment at the end of the final crop removal, without any impairment on plant growth. The better P stabilisation at the end of Alum-4 treatment was closely correlated with a progressive supply of Al³⁺ and a gradual decrease of pH, which resulted in higher contents of poorly-crystalline Al, Fe and exchangeable Ca. These aspects were conducive to increasing the soil P stabilisation and phosphate sorption. In terms of management, growers in continuous cropping systems could utilise split alum applications as a strategy to alleviate P losses in high-P enriched calcareous soil.

Introduction

Phosphorus (P) accumulation due to high P inputs has been a considerable issue in the arable land of China (Zhang et al., 2019). While recommended threshold soil test P (STP) values based on yield response to applied P (here these values are termed as agronomic threshold) have been identified for optimum crop production, many regions with intensive agriculture have STP concentrations above these agronomic threshold values (Gourley et al., 2015; Yan et al., 2013). Although it is questionable whether P fertiliser application needs to be continued in these intensive soils (Sharpley et al., 2013), there is a consensus that intensive agriculture is the main nonpoint source of P losses to surface waters (Giles et al., 2015; Kalkhajeh et al., 2018; Kronvang et al., 2005).

In calcareous soil which is widely distributed in the north of China, high base status and pH render most of the exogenous phosphate ion from fertiliser unavailable and transform it into various precipitated P forms, such as stable Ca-P and Mg-P (Yan et al., 2018). However, previous studies demonstrated that the proportion of the soil labile P increases abruptly when legacy P continues to accumulate in soil, specifically in soils fertilised with livestock manure (Withers et al., 2017; Yan et al., 2013). In China PVFs are utilised to spread large amounts of livestock manure (Yan et al., 2016). It was estimated that PVFs, which account for 12% of the total arable land in China, annually receive over 50% of the manure produced in China's livestock industry (Jia et al., 2018). Consequently, P losses in the dissolved as well as in the particulate form along surface and subsurface pathways were commonly reported in the soils of PVFs (Kang et al., 2018; Shi et al., 2008; Yan et al., 2016). It is reported that the STP concentrations at the depths of 60 cm in many PVFs soils still exceed the threshold STP values (Chen et al., 2019; Kalkhajeh et al., 2018; Yan et al., 2018). Best management practices (BMPs) to protect water quality involve matching crop requirements with P inputs. However, such options do not mitigate against legacy P in the soil (Withers et al., 2015). Soil amendment with P fixation materials (PFMs) has been researched and shows promise as an above baseline BMP for growers in areas of China with historically high application rates of fertiliser (Fan et al., 2020).

Alum amendment of soils has been shown to reduce P losses from the intensively fertilised horticultural soils, or soils that have received animal wastes in the last decade (Anderson et al., 2018; Fan et al., 2019; Lombi et al., 2010; Zhao et al., 2018). Peak et al. (2002) reported that at pH >6, alum precipitates out as poorly-crystalline Al hydroxides and then reacts with P via an adsorption mechanism as proven by X-ray absorption near edge structure (XANES) spectroscopy. A recent study of Fan et al. (2020) demonstrated P sorption on poorly-crystalline Al hydroxides mainly occurred through the displacement of inner-sphere Al-OH rather than a formation of AlPO₄ at pH 6.5. The maxima of adsorbed P on the poorly-crystalline Al hydroxides varied from 23.9 to 143 mg/g in the batch P sorption experiment (Gypser et al., 2018; Li et al., 2013; Wang et al., 2019). However, Fan et al. (2020) found that a calcareous soil amended with 20 g/kg of alum only had P sorption maxima of 0.45 mg P/g soil after 24 hrs. This raised questions about the decline of the P sorption amounts on Al hydroxides in the soil amended with alum. Batch studies exploring P sorption on the surface of Al hydroxides have identified that adsorbed P reached maxima and maintained stable after approximately 72 hr (Li et al., 2013; Wang et al., 2019). However, P cycling from soil non-labile P to labile P might be occurring to enable a new soil P balance when the soil labile P was fixed by Al hydroxides. It was reported that the time scale of P cycling operates from seconds to months from soil non-labile P to labile P (Helfenstein et al., 2018). In addition, other anions or soil colloids also react with Al hydroxides at the complicate soil system since Al also forms strong complexes with OH⁻, F⁻ or organic substances containing carboxylate and phenolic functional groups (Arai and Sparks, 2007; Essington, 2015).

Based on the evidence put forward above, the “little and often” approach of alum applications (multiple smaller applications of alum that account for the same amount of alum applied in one single application) may be a good strategy to increase the soil P stabilisation, as “little” can diminish the reaction between other anions and Al³⁺ whilst “often” can strengthen the reaction between the soil labile P and Al hydroxides. In particular, the split alum applications approach was recommended to maximise binding efficiency and minimise risk and treatment cost on the restoration of lake eutrophication (Kuster et al., 2020). However, the higher P stabilisation efficacy of alum in soils before crops are sown needs to be validated to elucidate a more effective management strategy for growers.

In this study, the hypotheses was as follow: P fixation efficiency would increase with an increase in the P sorption capacity induced by the alum split applications. To meet the objectives, a 15 month pot study with continuous cropping of maize-celery-tomato-tomato was established to (1) investigate temporal variations in soil labile P and transformation of the soil P forms and pools across single (20 g/kg) and split (4 × 5 g/kg before each sowing period) alum amendments; (2) differentiate the dominant anions competing with P in the alum amended calcareous soil.