The high base saturation under horticultural land uses as compared to < 23% for the pine forest and tea plantation soils is associated with the presence of soluble salts derived from lime, horse manure and Kfertilizer application. These soluble salts derived from the agricultural amendments are beneficial to soil fertility as they can be readily taken up by roots to meet plant nutrient requirements. Conversion of pine forest to intensive horticultural crops resulted in the increase of nitrate content by 4–7 fold . This high concentration of nitrate is explained in part by mineralization of horse manure and urea applications and the presence of positive charge on surfaces of nanocrystalline materials to retain anions. According to Auxtero et al. the positive charge of subsurface allophane-rich horizons allowed Andisols to retain mobile anions such as nitrate,hydroponic barley fodder system which is beneficial for crops. Further, the higher pH values of the IH soil may contribute to more favorable conditions for nitrification leading to the lower NH4 + and higher NO3 – concentrations found in the profile. Similar results were reported following forest timber harvest where soil NO3 – increased up to 8-fold shortly after harvest as compared to pre-harvest conditions . Previous studies measured anion exchange capacity of allophane-rich soils ranging from 0.4 to 12.2 cmolc kg−1.
This range of AEC values corresponds to 56 to 1700 mg NO3-N kg−1 , which appears sufficient to accommodate the KCl-extractable NO3 – concentrations that range up to 35 mg NO3-N kg−1 in the IH profile. Evaluation of N content in Java Island, Indonesia with different soil types and land uses showed higher soil N content in Andisols was associated with the presence of nanocrystalline materials . Retention of NO3 – within the soil profile reduces nitrate leaching and provides a readily available N supply for deeply-rooted crops . Under pine forest vegetation , the soil P retention was consistently high throughout the entire pedon . In contrast, the IH land use receiving application of horse manure for the past7 years showed appreciably lowering P retention in the upper 40 cm. The decrease in P retention and the increase of available P in the upper horizons of the IH profile were related to application of horse manure and inorganic SP36 fertilizer . These P dynamics could be associated with competition between organic functional groups derived from the horse manure and the applied P for sorption to the hydroxyl functional groups of the allophanic materials. Organic matter functional groups may block some reactive functional groups on allophanic materials, which in turn reduce P retention. In addition, the increase in pH from 4.5 in the PF soil to pH 6.1 in the topsoil of the IH soil may contribute to reduced P retention. This is supported by negative correlation coefficient between P retention and soil pH . Maximum phosphate sorption in Andisols often occurs in the pH range of 3.0–4.5 and decreases with increasing soil pH . Thus, the application of animal manure and lime appears to be an effective nutrient management strategy to enhance P availability in these high P fixing Andisols. Higher extractable S concentrations in the PF soil may be due to a combination of enhanced capture of H2S/H2SO4 emissions by the canopy of the pine forest, low S uptake by the pine forest and/or low soluble PO4 concentrations that could displace sorbed SO4. The depth trend for extractable SO4-S consisted of lower concentrations in the topsoil than the subsoil for pine forest and horticultural land uses.
This is related to competition with P and organic matter and with the increase in soil pH for horticultural crops . Previous workers have reported that sulfate and phosphate compete for the same anion-binding sites but P is adsorbed stronger than sulfate due to phosphate ions being able to form very strong inner-sphere complexes . In contrast, sulfate forms weaker inner-sphere and outersphere SO4 sorption complexes on short-range ordered materials, with the former becoming more dominant with decreasing pH and increasing sulfate concentrations . Pigna and Violante reported phosphate sorption 2–5 times greater than sulfate in Andisols and by increasing pH, phosphate sorption slightly decreased, whereas sulfate retention decreased dramatically . In addition organic matter competes more effectively with sulfate than with phosphate for sorption sites , resulting in low S availability in the topsoil horizons with high organic matter in the present study. Micro-nutrient availability is typically greater in more acidic soils due to higher metal solubility. In the present study, however, the micro-nutrient availability was higher in the horticultural soils having a higher pH . In particular, the addition of horse manure appears to provide both a source of micro-nutrients as well as high dissolved and particulate organic matter concentrations to enhance metal solubility by complexation. Therefore, manure additions appear to provide a strong benefit with respect to micro-nutrient availability for agronomic crops.The Andisols in this study contained much higher C stocks to a depth of 1 m as compared to the global average for tropical Oxisols and Ultisols of 9.7 and 8.3 kg m−2 , respectively . Further comparison to Oxisols and Ultisols from the Brazilian Amazon had C stocks from 8.5 to 10.5 kg m−2 , which were 2–3 time lower than the tropical Andisols in this study.
These comparisons indicate that Andisols have substantially higher capacity than other mineral soils to preserve organic matter. These results are consistent with those of Torn et al. who concluded that Andisols contain about twice as much organic C per m2 than Oxisols or any other soil orders, except for Histosols and Gelisols. Oxisols and Ultisols are dominated by low activity clays that provide less active mineral surfaces for physical and chemical stabilization of soil organic C . In contrast, N stocks of our tropical Andisols were similar in magnitude to Oxisols and Ultisols in the Brazilian Amazon that varied from 0.71 to 2.3 kg N m−2 , but mostly from 0.7 to 1.3 kg N m−2 in the upper 100 cm . Therefore, the Andisols of this study appeared to store organic matter with a higher C/N ratio than Amazonian Oxisols and Ultisols. Overall, soil carbon and nitrogen stocks in the upper 1 m of soil profiles increased in agricultural soils compared to the pine forest soil . These data appear to suggest degradation of soil organic C and N in the topsoil following conversion to agriculture but compensation by the elevated C and N in sub-soils. This condition results from pedon redistribution of organic C concentrations from topsoil to subsoil horizons. This redistribution may be attributed to a decrease in surface litter under agricultural land use with deeper incorporation of organic matter by tillage,livestock fodder system and/or deeper rooting system of some horticultural plants. Alternatively, the appreciably higher bulk densities of the agricultural soils contributed to higher organic C stocks compared to the pine forest soil suggesting a role for compaction in increasing C stocks on volumetric basis. Finally, it is possible that periodic volcanic ash deposits have resulted in burial of organic-rich horizons, leading to the high organic matter in subsurface horizons. Importantly, in spite of intensive agricultural production for > 100 years, there was no appreciable loss of organic matter from these soils as has been documented in many soils following conversion of forest vegetation to agricultural purposes. Similarly, Panichini et al. reported that disturbance of Andisols in Chili by forest management did not alter carbon storage. They posited that organic matter was stabilized by amorphous materials and organo-mineral complex formation, and the humid climate protected soils from irreversible drying and potential carbon loss. The ability of Andisols to strongly sequester and preserve organic C under various land-use/land management practices was demonstrated by the increase of organic matter in subsoil horizons of agricultural soils compared to the forest soil. In contrast, the lack of an organic matter build up in the topsoil and IH soil receiving horse manure for the last 7 years relative to the FH soil indicate that the added horse manure is quickly mineralized to provide nutrients to the horticultural crops. In addition, the increased N content from inorganic fertilizer may accelerate mineralization of organic C. On the other hand, the zero tillage in the FH soil contributed to the buildup and preservation of organic matter in the FH soil compared to intensive cultivation in the IH soil. The strong correlation between organic C and Alp and the lack of a significant correlation between organic C and Sio suggest that Al-organic complexes are more important than allophane in preserving organic matter in these tropical Andisols.
Microbial biomass C trends showed a positive relationship with total C and extractable DOC. The most evident change with respect to land use was the large decrease in MBC in the topsoil upon conversion from pine forest to agricultural production . Surprisingly, the lowest MBC values were found in the IH soil which received regular additions of horse manure for the past 7 years. Extractable DOC is considered an important carbon source to the microbial community and often correlates with microbial biomass. Extractable DOC represented 1.2–1.6% of total soil organic C for the PF and TP compared to < 1% for the IH and FH soils. This suggests that changes in vegetation possibly resulted in changes to the chemical nature of the organic matter affecting DOC solubility, which may affect substrate availability for the microbial community. Overall, agricultural practices had a strong impact in reducing microbial biomass C in topsoil horizons as compared to the pine forest. The microbial-labile pool of organic C is revealed by C mineralization rates during the incubation period. The overall CO2 mineralization rates followed PF > TP > IH > FH in both topsoil and subsoilhorizons . This agrees well with the highest DOC concentrations found in the PF soil and indicates more easily decomposable organic C substrates were available in PF soil than agricultural land uses. Interestingly, CO2 emissions shifted to IH > PF > TP > FH after day 70 in the topsoil, indicating depletion of easily decomposable C in the PF and TP soils. The much lower C mineralization rates in the subsoil than topsoil horizons were accounted for in part by the higher amorphous material content in the former . Chevallier et al. measured transformation of organic matter in volcanic soils by CO2 respiration and showed that the decomposition decreased as the soil allophane content increased. The low C mineralization rates for the FH profile is likely due to depletion of the microbial labile C pool as new organic carbon inputs were minimal over the last 7 years due to fallowing of the soil.This suggests that the topsoil contains more labile C substrate than subsoil horizons. According to Kavdir et al. , the fresh litter contained labile and easily decomposed materials, which mainly consisted of O-alkyl C. Inputs of new organic matter will be preferentially incorporated into the topsoil horizons and organic matter in the subsoil horizons is likely more strongly stabilized by physical and chemical mechanisms. The formation of metal–humic complexes was shown by positive linear correlation between dissolved organic C with Al- and Fe- extracted by Na-pyrophosphate . Determinant coefficients for Al and Fe were 0.84 and 0.80, respectively, suggesting that about 80% of dissolved organic C was bonded to the short-range ordered materials. The fraction of soil organic C bonded to Al and Fe varied from 25 to 50% with the magnitude following TP > FH > PF > IH in the topsoil and middle portions of the profiles . In contrast, the organic carbon bonded to metals in the lower pedon followed: IH > TP ∼ PF > FH. Previous studies on mineral control of carbon pool in Andisols in the Réunion Island showed the largest proportion of organic matter occurred as organo-mineral complexes . The lower metal-humus complexes of Andisols in the present study as compared to Andisols from Réunion Island could be associated with the higher annual rainfall and temperature under tropical conditions that accelerated organic matter decomposition. In our study the negative correlation between soil pH and Fep and Alp was observed indicating favorable conditions for organo-metal complex formation under acidic conditions .