High root expression of GS1 in these fields indicates that root N assimilation was elevated and thus actual plant N availability and uptake was higher than low inorganic N pools would suggest . Fields from group 2 demonstrated N surplus, showing similar yields to group 3 but with lower total and labile soil C and N and a higher potential for N losses, given much higher soil inorganic N . While actual N losses depend on a host of factors , high soil NO3 – is considered an indicator for N loss potential. Results from a companion study support the idea that soil microbes were C rather than N-limited in these fields, showing higher potential activities of C-cycling soil enzymes but low activities of N-cycling soil enzymes, the inverse of group 3 . An alternative multivariate clustering approach based on an artificial neural network suggests multiple potential drivers of higher inorganic N pools in these fields, including both management factors and soil characteristics . For instance, field 4 had strong indications of surplus N driven at least in part by a large application of seabird guano , a readily-mineralizable organic N fertilizer, at tomato transplanting when plant N demand is low. In contrast, higher inorganic N in field 8 was likely driven by low plant N demand based on very low soil P availability, which resulted in plant P limitation. These site-specific problems were identifiable due to the focus on variability across similar organic fields and illustrate the need for site-specific approaches to reduce N losses. Finally, the two fields included in group 1 were exemplary of N deficiency, in which low N availability compromises crop productivity but also likely limits N losses within the growing season. While low soil NH4 + and NO3 – concentrations were similar to group 3, low total and labile soil organic matter and poorly-timed organic matter inputs compromised microbial activity and likely limited N mineralization.Cytosolic glutamine synthetase GS1 encodes for the enzyme that catalyzes the addition of NH4 + to glutamate,fodder systems for cattle the former resulting from either direct uptake of NH4 + from soil or reduction of NO3 – in roots.
GS1 is thus the gateway for N assimilation in roots and is upregulated to increase root N assimilation capacity. Similar levels of GS1 expression in groups 2 and 3, in spite of large differences in soil NH4 + and NO3 – concentrations at the anthesis sampling, suggests that plant N availability is indeed higher in group 3 fields than would be expected based on measurement of inorganic N pools alone. The low levels of GS1 expression found in fields with clear N deficiency supports this idea. These results complement recent experimental approaches that showed rapidly increased expression of GS1 in tomato roots in response to a pulse of 15NH4 + -N on an organic farm soil, which was linked to subsequent increases in root and shoot 15N content, even when this pulse did not significantly change soil inorganic N pools. GS1 transcripts and glutamine synthetase enzyme activity also increased with increasing NH4 + and NO3 – availability in sorghum roots, suggesting this response may be widespread among plant species. Interestingly, inclusion of soil GWC in multiple linear regression models increased the proportion of GS1 expression variability explained to nearly 30% ; soil water content increases microbial activity as well as the mass flow and diffusion of inorganic N to roots. Further research will undoubtedly show how other factors like crop physiological N demand relative to C fixation and P availability increase the interpretability of N uptake and assimilation gene expression in roots.The N cycling scenarios identified on this set of organic fields corresponded at least in part with landscape clusters based on landscape and soil characteristics . Fields that balanced high yields with low potential for N loss and high internal N cycling capacity were part of PAM cluster 1, which had the highest productive capacity rating . Landscape clusters encompassing more marginal soils included both low-yielding fields exhibiting N deficiency or high-yielding fields that used inputs of highly available N like seabird guano to alleviate N deficiency . But these inputs led to the highest soil NO3 – levels and thus came at the cost of higher potential for N loss. Long-term efforts to increase internal soil N cycling capacity would help alleviate both N deficiency and the need for such large inputs of labile N. Whether farmers are willing to invest in management to increase soil N cycling capacity depends in part on how likely they perceive the benefits to be, especially on marginal soils.
The discussions that we had with each farmer in this study indicated genuine interest in adaptive management to further tighten plant-soil N cycling, but this may not always be the case. Indeed, the proportion of management vs. inherent soil characteristics responsible for driving differences in N cycling is challenging to untangle. Farmers may allocate more resources to more productive land and likewise fewer resources to more marginal land, or may selectively transition more marginal land to organic management. Documenting the multiple services provided by increases in soil quality and facilitating information exchange among organic growers such as through the landscape approach used here may help build momentum for efforts to improve soil quality and plant-soil-microbe N cycling.Net tropical forest loss of 7 million hectares per year occurred between 2000 and 2010, with conversion to agriculture accounting for 86% of deforestation . Annual deforestation in tropical Asia during the 1990s reached up to 5.6 million ha yr−1 , resulting in the emission of 1.0 Pg C yr−1 to the atmosphere . In Indonesia, the total forest area of 117 million ha in 1990 dropped to 89 million ha in 2011–2012 with primary, secondary and plantation forests occupying 45.2, 40.8 and 3.0 million ha, respectively . The average forest loss of 1.3 million ha yr−1 from 1990 to 2012 resulted from burning and conversion to agriculture, mining and infrastructure with Indonesia contributing to ∼10% of total global forest loss each year. Short-term changes in soil properties following conversion of tropical forests to agricultural land use are often pronounced and in most cases detrimental to sustainable agricultural production. In contrast to the Amazon rainforests supported by Oxisols and Ultisols , Indonesia’s rainforests are largely supported by volcanic soils, primarily Andisols. These Andisols support high agricultural productivity with some of the world’s highest human-carrying capacity being found on volcanic soils in Indonesia . With respect to greenhouse gases, fodder sprouting system Andisols are notable for having the highest soil carbon storage capacity among the mineral soil orders in temperate and tropical climatic regimes with an average carbon stock of 25.4 kg C m−2 . Matus et al reviewed soil carbon storage and stabilisation in andic soils and concluded that the most important mechanism of sorption of soil organic matter by short range ordered amorphous minerals is the ligand exchange.
While short-term changes in properties of tropical rainforest soils have been extensively studied, there is a paucity of information concerning long-term changes in soil properties resulting from changing land use and management practices, especially with respect to Andisols. Greenhouse gas emissions from agriculture are reported to contribute up to 30% of anthropogenic emissions . Soils can be a major source or sink of GHG from terrestrial ecosystems depending on the ecosystem disturbance regime and soil management practices. Soil carbon storage is dependent on soil mineral constituents, with volcanic ash soils typically having exceptionally high potential C stocks owing to their high content of active Al and Fe constituents . In Andisols, Chevallier et al. showed organic matter transformation to CO2 via microbial respiration was lower as allophane content increased. In addition, changes in land use/land cover alter organic matter quantity and quality, which are major factors controlling soil microbial biomass and activity . Given the high C stocks in Andisols, it is important to assess the fate of soil C following land-use conversion from forest to intensive agricultural production, especially with regard to rapid deforestation in the tropics. Andisols have several unique properties that affect agricultural productivity, such as high P fixation, high organic matter concentrations, a clay-size fraction dominated by pH dependent variable charge, low bulk density, high porosity, high water retention capacity and high mesopore content . In particular, high P retentionin Andisols can limit agricultural productivity by limiting plant availability of P. Currently, there is little information on how P retention and availability in tropical Andisols change with different land use and agricultural practices. Nitrate leaching characteristics in Andisols are also strongly affected by variable charged constituents as positive charges can retain nitrate enabling higher plantutilization efficiency. In southern Chile, Huygens et al. reported NH4 + and NO3 − retention of 84 and 69% of N fertilizer additions, respectively, after one year based on 15N pool-dilution and 15N tracer studies of forested Andisols. In Japan, the maximum nitrate adsorption by Andisols ranged from 0.4 to 7.0 cmolc kg−1 with the highest values occurring in soil horizons with high allophane content and low organic carbon content . Furthermore, Deng et al. evaluated the denitrification rates from eight Andisols under three different cropping systems in an intensive livestock catchment of central Japan and reported that N loss via denitrification from upland fields was almost negligible in spite of substantial N inputs . In addition to retention of NO3 − by positively charged colloids, a laboratory study by Matus et al. reported high retention of NO3 − in Andisols through transformation of NO3 − to dissolved organic nitrogen . In Indonesia, land use/land cover of Andisols is primarily native rainforest, tea plantation, horticultural crops, terraced paddy fields and other food crops. Land-use conversion from tropical rainforest to agriculture has taken place over long periods of time ; however, no rigorous studies have examined changes to Andisol soil properties over these time periods. In addition, several studies have examined microbial biomass carbon and CO2 measurements in topsoil horizons, however, MBC and CO2 measurements in subsoil horizons have been ignored although these measurements are crucial for explaining the exceptionally high carbon stocks in Andisols. Given the several unique properties of Andisols, it may be expected that these soils are more resilient to land-use change and agricultural management practices. Therefore, we hypothesize that the unique soil properties of Andisols lessen the negative impacts of land-use change from tropical forest to agriculture on soil physical, chemical and biological properties. The objective of this study was to take advantage of long term, land-use/land management changes to examine changes in several physical, chemical and biological properties of Andisols in tropical Indonesia following conversion of rainforest to tea plantation and horticultural crops.Samples were pretreated with H2O2 to remove organic matter and dispersed with dilute Na-hexametaphosphate. Silt- and clay-sized fractions were measured after sedimentation according to Stokes law. The sand fraction was separated from the clay and silt fractions by wet sieving through a 0.05 mm sieve. Water retention at various tensions was determined using a pressure plate. Plant-available water holding capacity was estimated as the volume fraction of water retained between 33 and 1500 kPa. A sample of < 2-mm , air-dry soil was placed on a porous ceramic plate and wetted by capillary action; gravimetric water content was measured following attainment of equilibrium at 33 and 1500 kPa. Soil pH was measured 1:2 in H2O and 1.0 M KCl. Phosphate retention was determined using the method of Blakemore et al. and the Bray-1 extraction was used as an estimate of available P . Exchangeable cations were displaced by 1 M NH4OAc at pH 7.0, then the cations were measured in the supernatant using an atomic absorption spectrometer . The cation exchange capacity was determined in 1 M NH4OAc after extraction of NH4 + by 10% NaCl as a measure of CEC. Base saturation was calculated as the sum of base cations by 1 M NH4OAc divided by CEC. Sulfate-sulfur was extracted using monocalcium phosphate as outlined by Schulte and Eik and available micronutrients were determined by DTPA extraction . All weight percent data were reported on an oven-dry basis .