Georgia is the top broiler-producing state in the United States of America, with a production of 1.4 billion birds in 2008 [1]. Assuming an average litter production of 1.5 kg per bird [2,3], Georgia is estimated to have produced 2.1 million Mg of poultry litter (PL) in 2008. Consisting primarily of poultry manure, bedding material, feathers, and some wasted feed, PL provides both macro and micronutrients to crops. Much of this PL is surface applied to pastures and no-till fields, or incorporated into the soil in conventional-till fields as a fertilizer. While this material offers an inexpensive fertilizer source particularly for nitrogen (N) and phosphorus (P), its N value decreases when N is lost as ammonia (NH₃) gas, especially from surface applications. This process, known as NH₃ volatilization, accounts for 4 to 60% of the total N lost from PL under laboratory conditions [4–6]. Ammonia loss from PL is important not only agronomically, but also environmentally. Deposition of NH₃ from the atmosphere can lead to N loading of lakes, indirect acidification of soils of low buffering capacity through nitrification, and damage of sensitive crops such as tomato, cucumber, and conifers [7–10]. In addition to the potential NH₃ impact, applying PL to soil may contaminate surface runoff with P [11]. Amending PL with acidified biochar may be useful to reduce such losses of N and P from PL. Reduction in the amounts of volatilized NH₃ may be achieved with biochar by decreasing the pH of the litter and by providing more exchange sites for ammonium (NH₄ ⁺). Reduction in the amounts of lost P may be achieved by the biochar’s capacity to adsorb phosphate [12].

Biochar is a black solid byproduct produced when biomass residues are converted to liquid and gaseous fuel through pyrolysis. It is composed primarily of polyaromatic carbon (C) [13–16], which suggests high resistance against decomposition [17–20]. Biochar has gained increasing interest as one of the means for sequestering carbon dioxide (CO₂) from the atmosphere. Lehmann [21] argues that when combined with bioenergy production, heating plant biomass can be a clean technology while at the same time sequestering carbon in biochar. When applied to soils, this byproduct has an estimated residence time of hundreds to thousands of years [19,21,22]. Adding to its refractory nature towards microbial breakdown, biochars are also reported to increase the cation exchange capacity (CEC) of soils [23,24]. The presence of biochars has been associated with the enhanced nutrient retention of Terra Preta soils that were formed originally from nutrient-poor, leaching-prone soils in the Central Amazonia [20,23]. The retentive property of biochars is also one of the major factors ascribed to better crop production with inorganic or organic fertilizer plus biochar combinations over inorganic or organic fertilization alone under tropical conditions [12,20,25,26].

Considering the environmental and economic benefits of renewable bioenergy over conventional burning of fossil fuels [21,27–30], it is anticipated that thermochemical generation of bio-fuel will be one of the alternatives to petroleum. In view of the anticipated increase in the supply of biochars in the future, utilizing this byproduct as a means for improving the retention capacity of poultry litter for nutrients such as N and P warrants an investigation.

The effect of biochar on the release of inorganic N and P has not been well investigated particularly in PL-fertilized soils. Assessment of the release of potentially available N and P is important for the resulting impact of acidified biochar-amended PL on both plants and the environment. Therefore, the objectives of this study were to evaluate the effect of acidified biochars on NH₃ volatilization and release of inorganic N and P from surface-applied or incorporated PL.

Surface-applied PL volatilized 226 μg N g⁻¹ soil whereas PLs amended with acidified biochars lost only 83 to 95 μg N g⁻¹ after 21 d (Table 2). Thus, adding acidified biochars reduced NH₃-N loss by 58 to 63%. No differences were observed between PLs amended with PC and PH. The reduction in NH₃ loss caused by the addition of biochar may have been due to a reduction in the pH of the PL + biochar mixture (Table 3), an increase in the hydrogen buffering capacity, and an increase in the cation exchange capacity (Table 1).

Poultry litter contains uric acid, which is converted to urea by the enzyme uricase. Urea is in turn hydrolyzed to ammoniacal N by the enzyme urease, with the process consuming H⁺ ions and raising pH [38]. Volatilization of NH₃ is a pH-dependent process, with conversion of aqueous NH₄ ⁺ into NH₃ starting around pH 7 [39]. Thus, decreasing the pH of the original PL from 8.55 to values of 7.26 to 7.39 by the addition of biochars (Table 3) likely decreased the conversion of ammoniacal N to NH₃, which helped reduce losses. In addition, the biochars provided H⁺ buffering capacity that likely helped resist increases in pH caused by urea hydrolysis (Table 1). Furthermore, biochars may have kept the amounts of volatilized NH₃ at lower levels by also retaining some of the NH₄ ⁺ on their exchange sites. If all of the exchange sites on the biochars were totally occupied by NH₄ ⁺, the theoretical maximum reduction in NH₃ volatilized from surface-applied PL could have been 79 and 71 μg N g⁻¹ soil by PC and PH, respectively (for PC: 2.09 g PC × 1 kg PC 1000 g⁻¹ PC × 17.4 cmol + kg⁻¹ × 0.14 g N cmol⁻¹ × 1000 mg N g⁻¹ N × 1 experimental unit (64.5 g soil) ⁻¹ × 1000 g soil kg⁻¹ soil = 79 mg N kg⁻¹ = 79 μg N g⁻¹ soil). However, these values could only account for about 55% (for PC: 79/(226 – 83) × 100 = 55%; for PH: 71/(226 – 95) × 100 = 54%) of the actual reduction in NH₃ loss observed by the addition of biochar in the surface-applied study (Table 2). Therefore, it is likely that the addition of biochar reduced NH₃ losses by a combination of pH decrease and increases in H⁺ buffering and cation exchange capacities of the PL + biochar mixture.

Incorporated PL without biochar lost 97 μg N g⁻¹ soil through NH₃ volatilization (Table 2). Although the treatments of surface-applied and incorporated PL were not applied in the same study and therefore cannot be statistically compared, it can be clearly seen that incorporated PL led to a smaller amount of NH₃ loss than surface-applied PL (226 μg N g⁻¹ soil). The smaller loss with incorporated treatments was likely due to (1) a lower amount of total released N (Table 2), possibly due to immobilization, and (2) a larger decrease in pH (Table 3), which would have reduced the amount of NH₃ in solution. For incorporated PL, taking into account the pH difference observed between the soil-alone treatment (pH = 6.69 from Table 1) and the pH of soil + PL (pH = 6.94 from Table 3), it is possible to use the H⁺ buffering capacity (Table 1) of the soil to calculate the amount of poultry litter N that would have been retained by the soil as NH₄ ⁺-N. Based on a H⁺ buffering capacity of 4 cmol H⁺ kg⁻¹ pH⁻¹ (Table 1) an increase in pH of 0.25 units would have created 1 cmol of negative charge kg⁻¹ (0.25 pH units × 4 cmol negative charge kg⁻¹ pH^–1), which could have retained a maximum of 140 μg N g⁻¹ soil (1 cmol NH₄ ⁺-N kg⁻¹ × 0.14 g N cmol⁻¹ × 1000 mg g⁻¹ = 140 mg N kg⁻¹ = 140 μg N g⁻¹). However, considering the possibility that (1) some of the released acid from the soil may have been consumed by the alkaline buffering capacity of PL (which was not determined in this study), and (2) some of the created CEC may have been occupied by other cations in the PL, the soil may have retained less than 140 μg N g⁻¹ soil. The difference between what could be potentially volatilized (based on the losses observed in the unamended surface-applied PLs [226 μg N g⁻¹ soil]) and the estimate of the amount of N that the soil could retain (<140 μg N g⁻¹ soil) would indicate that >86 μg N g⁻¹ soil (226 – 140 = 86) could still be lost from the incorporated unamended PL. This estimate is not far from the 97 μg N g⁻¹ soil that was actually volatilized from unamended incorporated PL. Therefore, insufficient H⁺ buffering capacity from the soil used in this study may have been the reason why some NH₃ was still volatilized from unamended PL even under incorporated incubation.

When acidified biochars were added, incorporated PL lost 39 to 43 μg N g⁻¹ as NH₃ after 21 d (Table 2), which corresponds to reductions of 56 to 60% when compared to unamended PL. The lack of differences in NH₃ loss among biochar-amended PLs may have been due to the biochars providing similar H⁺ buffering capacities (Table 1).

Total amounts of leached inorganic N tended to be greater from surface-applied and incorporated PL with acidified biochars than without these amendments, particularly for PH (Table 2). This was probably due to the reduction in NH₃ loss caused by the biochar addition, which would have left more inorganic N in the soil susceptible to leaching. When analyzing the constituents of inorganic N leached (NH₄ ⁺-N or NO₃ ⁻-N), however, large variability in the data prevented clear detection of an increased amount of leached NH₄ ⁺ (Table 4). Leached amounts of NH₄ ⁺-N or NO₃ ⁻-N in all PL + biochar treatments are assumed to be derived from PL because the initial amounts of inorganic N present in the acidified biochar were very low compared to that of PL (Table 1). It should be noted that the large amounts of leached NH₄ ⁺ may have been caused by the use of 0.01 M CaCl₂ as leaching solution because Ca₂⁺ (even at low concentrations) could have displaced cations from exchange sites. The use of deionized water as leaching solution would have likely resulted in lower amounts of leached NH₄ ⁺, but soil dispersion caused by deionized water prevented us from using it.

Inorganic P leached from biochar-amended PL was not significantly different from that of the unamended PL in surface or incorporated incubations (Table 2). In general, incorporated litter with or without acidified biochars tended to have more inorganic P leached compared to surface-applied treatments, although these differences could not be tested statistically because the surface-applied and incorporated studies were conducted separately. As shown in Table 3, litters unamended or amended with acidified biochars in the presence of soil (in the same proportion as those in the incorporated incubation) had lower starting pHs compared to those that were measured without soil (as in surface incubation). Lower pHs of incorporated compared to surface-applied litters may have caused more of the inorganic P from PL to solubilize [40] and become available for leaching.

Acidified biochars had no significant effect on KCl-extractable NH₄ ⁺-N, NO₃ ^–-N (data not shown) and PO₄ ^–3-P (Table 2) in surface-applied and incorporated PLs. Release of total inorganic N from biochar-amended PLs also showed no significant difference from unamended PL in surface-applied or incorporated incubations (Table 2). For surface-applied PLs, 62 to 68% of the total amounts of the released inorganic N (totaled from volatilized, leached and extracted N) was apparently derived from the initial inorganic N of the PL, whereas for incorporated PLs all the released inorganic N apparently came from its initial inorganic N. Negative values for the percentage of organic N mineralized from incorporated PL (data not shown) indicates immobilization of its mineralized N or possibly even some of its initial N. This may be the reason for lower amounts of total inorganic N released from incorporated compared to surface-applied PL. Nitrogen immobilization may have been due to the fact that the soil used in the study was collected from a pasture and although it was passed through a 2-mm sieve, it still contained very small particles of plant residues.

Several workers have reported that crop performance is enhanced when inorganic or organic fertilizers are combined with charcoal. This has been assumed to be partly due to the enhanced nutrient retention capacity of the soil with charcoal [12,20,26,41] particularly for leaching-prone soils in the tropics. For example, in their lysimeter experiment with rice, Lehmann et al. [42] observed increased proportion of N taken up by the plants relative to what was lost to leaching when charcoal was added to inorganic or organic fertilizers. In our study, however, acidified biochars did not reduce the amounts of leached or extracted inorganic N. This may have been due to the use of 0.01 M CaCl₂ as leaching solution or to the relative low concentration of biochar used. The amount of biochar used in our study was only 3.2% of the soil weight in each experimental unit (2.09 g biochar/64.5 g soil × 100 = 3.2%) compared to 20% of the soil weight used by Lehmann et al. [42].

Increased P availability has been reported in the presence of biochar [43–46]. The mechanisms suggested for biochar influence on P availability are change in soil pH, which then influences the interaction of P with other cations, or enhanced retention through anion exchange [46]. In this study, however, acidified biochar at the applied rates did not have a significant effect on soil P retention

Acidified biochars reduced NH₃ losses by 58 to 63% in surface-applied PL and by 56 to 60% in incorporated PL. Adding acidified biochars to PL generally had no effect on the amounts of leached NH₄ ⁺-N, NO₃ ^–-N and KCl-extractable N from surface-applied and incorporated PLs, except for a small increase in leached NH₄ ⁺ from PL + PH in the incorporated incubation. If leached and extractable N is considered potentially available to plants, then the lack of a biochar effect may be considered a positive effect for soils that are not susceptible to leaching. However, acidified biochars did not affect the amounts of leached- or KCl-extracted P, which indicates that these biochars may not be useful to reduce P loading in runoff from pastures receiving surface-applied PL.