Liming is commonly used to improve the productivity of acidic soils in agricultural systems. The addition of lime increases the availability of nutrients, which would otherwise be strongly limited by low soil pH¹. An increase in soil pH, as a result of liming, increases microbial activity^2–4, resulting in increased decomposition of resident SOM^5,6. The decomposition could result in increased mineral N and P and losses of SOC through CO₂ emission^7,8. Generally, increasing soil pH to >5.5 would be optimum for the development and growth of nitrifiers⁹. High rates of nitrification have been reported when lime is applied to strongly acidic soil¹⁰. Zhao et al.¹¹, Lyngstad¹² and Yao et al.¹³ reported increases in nitrate-N after application of lime to forestry soils. The increase in nitrification can be explained by high microbial activity following lime application attributed to an increase in soil pH^2–4,14. However, other results have shown liming to either have no effect¹⁵ or a decrease¹⁶ in microbial activity of highly acidic soils. These differences could be due to soil physico-chemical properties such as total C and N, C: N ratio and soil pH. The differences in microbial response could also be due to differences in lime requirements of the soils (e.g. soils with high acid saturation would require more lime) and also differences in organic matter contents. The response of microbial communities and mineralization process to liming also varies with soil type and management conditions⁵. No effects or decreased mineralization with lime application could be attributed to increased protection of organic substrates due to increase aggregates stability¹⁷. In addition to increased microbial activity, lime addition increases P availability in highly acidic soils¹⁸. The lime application increases P availability by decreasing Al³⁺ and Fe³⁺ ions and fixation on aluminium and iron oxides while over-liming to pH values up to 8.5 could result in precipitation of calcium phosphate¹⁹. While the role of Al-organic matter complexes in fixing P is not clearly understood, the decomposition of the organic matter may release the bound Al into the soil solution, due to liming, with the possible increase in fixation of P. This effect has not been ascertained, including in humic soils of South Africa.

Humic soils are intensely weathered and highly acidic and are characterised by high C (>1.8%) content, high buffering capacity, and good drainage, low bulk density and base status²⁰. These soils are restricted to regions receiving high rainfall and cool temperatures mostly in parts of the Eastern Cape, KwaZulu-Natal and Mpumalanga in South Africa²⁰. There is no evidence, in literature, of the occurrence of these soils other than in South Africa. The characteristics of these soils are relatively similar to those of Andosols but the conditions on which they are formed are different, and the organic matter concentrations in some humic soils are higher. From our preliminary experiments, some humic soils have as high as 10% organic C and such levels of organic matter could results in excessive mineralization of N with potential for leaching to ground water. Like in Andosols, formation of Al-organic matter and Fe-organic matter complexes²¹ is believed to be a major mechanism of soil organic matter (SOM) stabilisation in humic soils. These organo-mineral complexes increase the protection of SOM from microbial decomposition²². The effects of liming on nitrogen mineralization and P availability could be magnified in the highly productive humic soils.

A large portion of humic soils in KwaZulu-Natal is under sugarcane, where treatment with lime is used to reduce the effects of acidity. The reduction in acidity could increase microbial activity, decomposition of SOM and mineralization of nutrients, making a significant contribution to crop N and P requirements, while the release of Al bound to SOM could increase exchangeable acidity or moderate effects of lime. Depending on the concentration of mineral N from decomposition of resident SOM, crop requirements could be achieved, with possible excess N contaminating ground water. The good drainage of humic soils²⁰ increases the possibilities of any excess nitrates to be leached. This effect could be particularly important with increase in demand for arable land which could result in humic soils under forestry and grassland being converted to cultivation with consequent inevitable need for lime. There is a need to understand the decomposition of SOM in humic soils especially those under forestry, which could be converted to arable land. Considering the high C content stored within these soils²⁰, which could be altered following liming, there is a need to understand the effects of lime application on pH, concentrations mineral N and P in disturbed humic soils under forestry. The objective of the study was to determine the effects of liming on N and P availability from humic soils during incubation.

The increase in pH with an increase in lime rate (Fig. 1A) was due to an increase in hydroxide ions following lime application²³. The increase in pH supported greater microbial activity², resulting in greater decomposition of organic matter⁶, resulting in the increase in ammonium-N concentrations in the 0–7 and 0–14 days periods for the soils from Eston and Eshowe, respectively (Fig. 2A,B). An increase in pH upon the addition of lime removes acidity as a constraint, increasing microbial activities resulting in increased mineralization of N³. Anderson²⁴; Chagnon et al.²⁵ reported that an increase in soil pH after liming results in a significant increase of SMB-C. Similar results were reported by several authors^14,26–28. The increase in SMB-C coincided with the increase in nitrate-N (Fig. 3A) and with application rate. The decrease in pH of Eshowe soil could be due to decomposition of SOM releasing humic acids initially and Al¹⁹ and low level of lime applied (Fig. 1B) while lower ammonium-N concentrations was also available (Fig. 2B), than from the Eston soil (Fig. 2A), as SOM decomposed.

The highest concentration of ammonium-N at day 7 for the Eston (Fig. 2A) and 14 for the Eshowe soil (Fig. 2B) suggest that upon removal of pH constraints rapid decomposition of SOM and N mineralization occurred releasing ammonium-N. Baggs et al.²⁹ reported similar results, where ammonium-N was higher after the addition of lime due to increased mineralization rates. The decrease in pH between day 7 and day 28 (Fig. 1A) was attributed to nitrification, which released H⁺ ions. The sharp decrease in ammonium-N from day 7 in the Eston soil (Fig. 2A) and 14 for the Eshowe soil (Fig. 2B), corresponds with an increase in nitrate-N concentration (Fig. 3A,B). These findings are coherent with Fuentes et al.⁹ who reported an increase in nitrate-N with increasing lime application rate. This decrease in soil pH, while lime is applied, could also be explained by aluminium released from Al-organic matter complexes during decomposition of SOM, increasing exchangeable acidity. The nitrate-N concentrations in the limed Eston soil could be more than required by crops, and could possibly leach resulting in groundwater contamination. At triple the recommended lime rate, the Eston soil had a maximum nitrate-N concentration of 623 mg N kg⁻¹ soil, which is extremely high for crop uptake³⁰. For example, crops like maize require 220 kg N ha⁻¹ to produce 10 t ha^−1 31 while spinach requires 100 kg N ha⁻¹ for optimum yields³⁰. Conversion of humic soils from wattle forestry to arable crops, with lime applied at recommended rates, could possibly result in a concentration of 787 kg N ha⁻¹ (mineral-N) without any addition of inorganic fertiliser. Such excessive mineral-N concentrations as a result of liming could be lost as NO₃-N. While lime makes N more available for crops, excessive concentrations poses risks to the environment. Although the concentrations were extremely high under conditions of continuous optimum temperature and moisture during incubation, the levels could be lower under field conditions, where wetting and drying occur and where temperatures fluctuate. Potential leaching of nitrate-N from cultivated humic soils and its concentration in groundwater needs to be evaluated since these acidic soils are usually limed when used for cropping. Generally, most crops require between ~20–400 kg N ha⁻¹ for optimum yields except for some pastures, nuts and fruits which require very low N³⁰. The differences in nitrate-N between the two soils used could be explained by differences in lime requirements due to differences in acid saturation of >80% and <30% for Eston and Eshowe soil, respectively (Table 1). The soil from Eston also had higher organic C and N than that from Eshowe, further explaining the differences in the magnitude of nitrate-N concentrations. Furthermore, the Eston soils had pH >5.5 following lime application at triple rate while Eshowe had pH <5.5 which inhibits nitrifiers⁹. Higher ammonium and nitrate-N at 0–10 cm depth could be explained by the higher total C and N contents (Table 1) at a shallower depth, which acts as energy and nutrient source for microbes. These findings are also evidenced by the lower C: N ratio (Table 1) at depth 0–10 than 10–20 cm, supporting the higher mineralization rates that occurred at the 0–10 cm depth. The increase in ammonium-N concentration from day 56–112 (Fig. 2A) and day 56–84 (Fig. 2B) in the control could possibly be due to the death of microbes releasing N as ammonium-N back into the soil solution.

Acidic soils have high P fixation capacity due to high Al and Fe decreasing its availability³². The decrease in extractable P from day 0–28 (Fig. 4A,B) could be due to its immobilization by soil microorganisms and its fixation as Al- and Fe-phosphates and on Al and Fe oxide (Figs. 5B,C and 6B,C) in the highly weathered and acidic soil conditions. This is so, because addition of lime increases microbial communities², hence, increasing decomposition of SOM and release of Fe and Al¹⁹. The increase in extractable P between days 21 and 56 for the limed treatments, in the Eston soil, with 80% initial acid saturation, shows that microbial immobilization contributed to the decline in the first 21 days, and then released the P when the microbes died. Furthermore, the lowest observed extractable P coincided with highest Al and Fe bound P. Significant P fixation recorded for the control (decrease to almost zero after 28 days) suggests a concern of limited available P and its implications for the productivity of humic soils. After application of lime microbial activities initially increased and could have led to immobilization of extractable P. The decrease in Al-P and Fe-P could be due to their precipitation as insoluble Al(OH)₃ and Fe(OH)₃ after increased addition of liming material²³. In addition, Al and Fe oxides become more negatively charged with an increase in pH contributing to an increase in available P^33,34. The decrease in the fractions with an increase in lime rate could be due to soil pH >4 at day 112 in triple application rate for Eston soil (Fig. 1A) as maximum adsorption of P as Al-P or Fe-P occurs at pH <4¹⁸. As a result, Fe and Al bound P decreased as pH increased. Even at triple the recommended lime rate the Al-P, Fe-P and CBD-P were still >80 mg kg⁻¹, suggesting that such high rates did not eliminate the P fixation capacity of these soils. This could be because the recommendation rate did not account for Al from Al-organic matter complexes, which are released when the organic matter is decomposed and is hydrolysed to cause acidity. This corresponds with the high extractable P available at high application rates of lime after 28 days (Fig. 4A). Low P was fixed on calcium because of low calcium content in highly weathered soils²⁰. The increase in Ca-P with application rates of lime could be explained by an increase in calcium concentration after the addition of lime (calcium carbonate), especially at high pH. These findings are similar to Kiflu et al.³⁵ observation where Ca-P increased with lime application.

Application of lime to humic soils increases mineral-N (ammonium- and nitrate-N) and to a lesser extent, extractable P. Liming the strongly acidic humic soils at a recommended rate or higher released excessive amounts of nitrate-N when incubated under optimum conditions of temperature and moisture. Higher lime rates increased soluble P, microbial biomass C and N, and reduced P associated with Al and Fe and their oxyhydroxides. Even at triple the recommended lime rate, Al-P, Fe-P and CBD-P still occurred at >80 mg kg⁻¹. The increase in mineral-N and P could be sufficient to increase crop productivity, without the addition of fertiliser, at least in the short term. High mineral N as a result of liming could be associated with mineralization of C resulting in high CO₂ emissions and a decline in soil C in the long-term. Future work must be done under field conditions with up to double the recommended lime rates. Also, future research needs to be done on CO₂ emission and leaching of nitrates from humic soils after lime application, under field conditions, where temperature and moisture fluctuate, considering the possibilities of conversion of the forestry and grasslands on humic soils to arable land for crop production.

The incubation study was conducted under laboratory conditions at the University of KwaZulu-Natal, Pietermaritzburg campus (29° 37′ 32.9″ Latitude; 30° 24′ 18.8″ Longitude) in KwaZulu-Natal Province of South Africa.