Phosphorus release behaviors of poultry litter biochar as a soil amendment

https://doi.org/10.1016/j.scitotenv.2015.01.093Get rights and content

Highlights

  • The predominant portion of P in poultry litter biochar is water insoluble.

  • Poultry litter P was immobilized by forming Ca/Mg (pyro)phosphates in biochar.

  • Release of P from biochar was slower and steadier than from raw poultry litter.

  • Soil pH greatly influenced the P release patterns of poultry litter biochar.

  • Soil amendment with biochar instead of poultry litter would reduce P runoff risks.

Abstract

Phosphorus (P) may be immobilized and consequently the runoff loss risks be reduced if poultry litter (PL) is converted into biochar prior to land application. Laboratory studies were conducted to examine the water extractability of P in PL biochar and its release kinetics in amended soils. Raw PL and its biochar produced through 400 °C pyrolysis were extracted with deionized water under various programs and measured for water extractable P species and contents. The materials were further incubated with a sandy loam at 20 g kg 1 soil and intermittently leached with water for 30 days. The P release kinetics were determined from the P recovery patterns in the water phase. Pyrolysis elevated the total P content from 13.7 g kg 1 in raw PL to 27.1 g kg 1 in PL biochar while reduced the water-soluble P level from 2.95 g kg 1 in the former to 0.17 g kg 1 in the latter. The thermal treatment transformed labile P in raw PL to putatively Mg/Ca phosphate minerals in biochar that were water-unextractable yet proton-releasable. Orthophosphate was the predominant form of water-soluble P in PL biochar, with condensed phosphate (e.g., pyrophosphate) as a minor form and organic phosphate in null. Release of P from PL biochar in both water and neutral soils was at a slower and steadier rate over a longer time period than from raw PL. Nevertheless, release of P from biochar was acid-driven and could be greatly promoted by the media acidity. Land application of PL biochar at soil pH-incorporated rates and frequency will potentially reduce P losses to runoffs and minimize the adverse impact of waste application on aquatic environments.

Introduction

The U.S. poultry industry generates more than 11 million tons of litter wastes annually (Gollehon et al., 2001). Poultry litter (PL) contains high levels of organic carbon (OC), nitrogen (N), phosphorus (P), potassium (K), and other plant nutrients and is commonly applied to cropland as a soil amendment (Guo et al., 2009). In concentrated poultry production regions such as Delmarva Peninsula, however, repeated and excess application of PL to limited acreage of land has resulted in severe nutrient accumulation in soils and subsequent runoff and leaching losses to water bodies, causing water eutrophication and quality degradation (Boesch et al., 2001, DNREC, 2006).

Poultry litter has an average P:N ratio of 1:2, far greater than the typical crop removal rate of 1:6 (MSU Extension, 2011). Land application of PL at rates based on crop N requirements would furnish vastly excessive P and increase the risk of P input to natural waters (Harmel et al., 2004). Studies have indicated that losses of P from land-applied animal manure are determined primarily by the content of water-soluble P (Shreve et al., 1995, Hart et al., 2004). As an example, 12–20% of the P in PL was water-soluble and was lost within five rain events following land application (Guo et al., 2009). It would be advantageous to reduce the rate of P losses by converting PL into a more stable, slow-release source of nutrients. This would be an effective strategy to maintain a more constant and longer-term supply of nutrients in soil, preventing rapid nutrient losses in runoff and hence reducing the risk of eutrophication.

A potential approach to implement this strategy is to transform PL through pyrolysis and use the biochar produced as a soil additive. In recent years, biochar prepared from excess biomass has been suggested as a stable medium for long-term carbon storage (Lehmann, 2007), a beneficial by-product of bio-oil and syngas production (Ro et al., 2010, Guo et al., 2012), a feasible means to reduce volume and mass of organic wastes (Cantrell et al., 2007), and a soil amendment to not only improve soil quality and crop yield but potentially decrease emissions of greenhouse gases from soil (Lehmann and Joseph, 2009, Chintala et al., 2014a). For PL, conversion to biochar may also slow down nutrient release and thus minimize the environmental impacts of the poultry industry. As suggested by a recent study by Song and Guo (2012), conversion of PL into biochar through slow pyrolysis at ≥ 350 °C decreased the fraction of water-soluble P from 19.5% to below 6.9%. Pyrolysis of cattle manure (9.1 g P kg 1) at ≥ 350 °C reduced this fraction from 10.2% to 0.21% (Cao and Harris, 2010). Thermal treatments of sewage sludge (26 g P kg 1) at 400–800 °C in the presence of N2, CO2, or air altered the fraction from 20.7% in the feed to < 2.5% in the resulting char/ash products (Qian and Jiang, 2014). Chintala et al. (2014b) found that merely 0.34–0.51% of the total P (1.9–2.0 g kg 1) in biochars derived from corn stover and switchgrass was 0.5 M NaHCO3-extractable. Due to the richness of Ca2 + and Mg2 + (12–13 g kg 1) in the ash components, the corn stover and switchgrass biochars even demonstrated significant sorption capacities for P in aqueous solutions.

While the above result is encouraging, the overall lability and release dynamics of P in PL biochar as a soil amendment – which determine the rates of P supply for crops and the risk to contaminate water following land application – has not been well-characterized. This study was undertaken to evaluate the P water extractability of PL biochar, to characterize the P release kinetics of the material as a soil amendment, and to explore the mechanism for P immobilization during pyrolysis.

Section snippets

Preparation of soil and PL biochar

The soil used in the experiments was Sassafras sandy loam (fine-loamy, siliceous, semiactive, mesic Typic Hapludults) collected from the top 20 cm of a Delaware cropland on which soybean, corn, and winter wheat were growing in rotation. The soil was air-dried, ground to pass a 2-mm sieve, and evaluated for fertility characteristics. It contained 55.4 mg kg 1 Mehlich-III extractable P and 0.22 mg kg 1 water soluble P. Selected physical and chemical properties of the soil are given in Table 1.

Granular

Characteristics of PL biochar

Under the pyrolysis conditions, the mass of biochar produced was 51.5% that of the original PL dry mass. As a result of the reduction in total mass, the conservative elements were enriched in biochar, showing concentrations nearly 1.9 times that in the raw PL (Table 2). Virtually all P and the metallic elements Al, B, Ca, Cu, Fe, K, Mg, Mn, and Zn in PL were preserved in biochar, suggesting little loss of P or metals from PL occurred during pyrolysis at 400 °C. Hence, biochar is a richer source

Conclusions

Our results suggest that pyrolysis immobilized the water extractable P in raw PL by converting it into more stable, less available magnesium (and calcium) phosphate minerals in PL biochar. Compared to raw PL, biochar is a richer source of P on a dry mass basis, offers a higher liming capacity, and could potentially be a slow-release source that supplies P in soil at a slow and constant rate over a longer time period. The last characteristics may result in more efficient uptake by plants and

Acknowledgment

The research was made possible by the National Science Foundation EPSCoR Grant No. EPS-0814251 and the USDA-NIFA Grant No. 2010-38821-21568.

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