Impact of intensive vegetable production on the organic matter, nitrogen, and phosphorus levels of a volcanic soil (Andisol)

by Niezel Jane D. Estrellanes

Volcanic soils such as Andisols have many unique properties not usually found in soils derived from other parent materials (http:/www.ctahr.hawaii.edu). They are often very young and acidic depending on which type of volcanic materials they come from. They also largely consist of non-crystalline minerals such as allophane and imogolite (Nanzyo et al., 2002) which form strong bonds with organic matter thereby protecting it from decomposition. Andisols are usually light textured and are easy to till. However, they form strong complexes with phosphorus, rendering this element unavailable to plants.

One of the sampling sites in the central volcanic highlands of Ormoc, Philippines

This thesis research, a component of the Australian Center for International Agricultural Research (ACIAR) Soil Project based at the Visayas State University, evaluated the impact of intensive vegetable production on the nitrogen and phosphorus levels of volcanic soils in the central highlands of Leyte specifically in Cabintan, Ormoc City, Philippines. Twelve sampling sites (vegetable farms) including a reference site were chosen for this study. The objective was to find out if the continuous and heavy application of fertilizers for intensive vegetable production have caused the accumulation of nitrogen and phosphorous in Andisols.

Location of the sampling sites in Cabintan, Ormoc City

Soil samples were taken from the following soil depths 0-20, 20-40, 40-60, 60-80, and 80-100 cm using a soil auger. They were air-dried and passed through 2-mm sieve and analyzed for selected physical and chemical parameters such as pH (H2O, KCl, and NaF), available phosphorus, and particle size distribution. Enough 2-mm samples were also ground further and allowed to pass through 0.425-mm for organic matter content and total N analysis. 

Soil OM contents of the sampling sites. Sites 2, 4, 6 and 7 have much 
higher values than the reference site 12.

Soil N contents of the sampling sites. Most of the sites have much higher 
N levels than the reference site 12

Soil available P contents of the sampling sites. Reference site (12) has 
much lower P levels compared to all other sites.

Results revealed that the majority of the vegetable farms sampled showed higher amounts of organic matter (OM), nitrogen (N), and available phosphorus (P) than the reference site (secondary growth forest). This clearly indicates the impact of the high and continuous application of chemical and organic fertilizers (chicken manure). The very high nitrogen contents of the soils point out to nitrogen pollution which could have a serious negative impact on the groundwater quality. The high available phosphorus contents of the soils suggest improvement of the supply of this nutrient to the crops considering the fact that it is the major limiting nutrient to crop production in Andisols.

Based on the results it can be concluded that:

a. The intensive vegetable production has increased the nutrient status of the Andisol; 

b. The frequent and high rates of fertilizer application for the intensive vegetable production have greatly increased the nitrogen and phosphorus contents of the Andisol soil.

c. Nitrate and phosphate pollution of the groundwater and the nearby rivers may result from intensive vegetable production. 

Biological nitrogen fixation in corn

Corn (Zea mays L.) can establish rhizospheric or endophytic associations with various nitrogen-fixing bacteria (diazotrophs) such as Azospirillum, Klebsiella, Pantoea, Herbaspirillum, Bacillus, Rhizobium etli and Burkholderia. Most of these diazotrophs can grow in the intercellular tissue of plants without causing any disease.

Biological nitrogen fixation (BNF) is the biological process by which nitrogen (N2) in the atmosphere is converted to ammonia by an enzyme called nitrogenase. The screening of plant genotypes for their enhanced ability to acquire nitrogen by BNF can reduce the use of expensive nitrogen fertilizers in several important crops like sugarcane, rice, wheat and corn. It can greatly benefit particularly the poor farmers of developing countries.

In a recent study aimed to quantify the symbiotic biological nitrogen fixing activity of a range of commercial corn cultivars, Montanez et al. (2009) demonstrated that corn cultivars obtain significant nitrogen from BNF, the level of which varied with corn cultivar and nitrogen fertilization level. The study showed that some cultivars were more sensitive than others to nitrogen application and that 15N isotope dilution method is a useful tool to screen and select corn cultivars with any potential BNF.

Reference

Montanez A, Abreu C, Gill PR, Hardarson G, and Sicardi M. 2009. Biological nitrogen fixation in maize (Zea mays L.) by 15N isotope dilution and identification of associated culturable diazotrophs. Biology and Fertility of Soils 45: 253-263

Effects of biosolids application on N mineralization

Sewage sludge is the solid, semi-solid or liquid residue generated during the treatment of domestic sewage. Biosolids are the treated form of sewage sludge. The use of biosolids as soil amendments is widely seen as a way to reduce the accumulation of wastes and at the same time to enhance soil fertility for crop production. Studies have shown that the use of biosolids as soil amendment is an effective means of recovering plant nutrients and improving the physical and microbiological properties of soils.

However, there are problems associated with the use of biosolids such as heavy metal contamination and nitrate pollution. Biosolids containing excessive levels of heavy metals should not be used as soil amendments. As for nitrate pollution due to excessive mineralization, Hseu and Huang (2005) proposed that this maybe avoided by regulating the annual rate of application of biosolids to soil based on crop N requirement and the anticipated net amount of organic N mineralized in the soil treated with biosolids.

In an interesting study aimed to characterize the influence of the application of biosolids on the soil potential for N mineralization (N0) and also to elucidate the kinetics of N mineralization in tropical soils treated with different biosolids, Hseu and Huang (2005) used anaerobic biosolids and aerobic biosolids obtained from the wastewater treatment plants in Kaohsiung and Taipei, Taiwan. The biosolids were applied at rates of 10, 50 and 100 Mg ha−1 to three tropical soils and incubated for 48 weeks.

Findings of the study revealed that addition of both kinds of biosolids to the soil increased N mineralization potential to an extent related directly to the application rate and the N content of the biosolids. However, the cumulative amounts of N mineralized for soil treated with aerobic biosolids greatly exceeded those for the soil treated with anaerobic biosolids. Sandy soil treated with biosolids exhibited a relatively low potential for mineralizing N.

The contamination of the biosolids with relatively high levels of heavy metals such as Cu and Zn did not prevent an increase in N mineralization resulting from the application of biosolids to the soils. Approximately 3–34% of the total N content in the biosolids-treated soils was mineralized for 48-week incubation. Based on a demand of 150 kg N ha−1 for vegetable production in Taiwanese soils, the rate of biosolids application in the three soils are safe and will not cause nitrate accumulation.

Reference

Hseu ZY and CC Huang. 2005. Nitrogen mineralization potentials in three tropical soils treated with biosolids. Chemosphere 59: 447-454.

Concentration affects plant uptake of inorganic and organic forms of N

It is now recognized that plants take up N from the soil in three forms: nitrate, ammonium, and amino acids (dissolved organic N). Although scientific evidence on plant uptake of amino acids has existed in the last few decades, it is only recently that the contribution of amino acids to plant nutrition has been recognized (see Warren 2009 and literatures cited). So the traditional view that organic N has to be mineralized first into nitrate and ammonium in order to be available to the plant is not anymore valid.

Different plant species vary in their preference for N forms. For instance, early successional plant species are known to have a higher capacity for nitrate uptake than late successional species. Uptake of N in the form of ammonium and amino acids is thus more important for the latter species. In a recent study to test the hypothesis that substrate concentration affects plant preference for N forms, Warren (2009) used the herb Ocimum basilicum and the evergreen tree Eucalyptus regnans. He placed roots of intact seedlings in equimolar mixtures of nitrate, ammonium and glycine (amino acid). His results revealed that substrate concentration influenced the preference of both plants for N forms. This means that whether the plant prefers one N form over another (e.g. nitrate over ammonium and amino acid or vice versa) depends on their concentrations in the growth medium or soil.

Reference

Warren CR. 2009. Does nitrogen concentration affect relative uptake rates of nitrate, ammonium, and glycine? J. Plant Nutr. Soil Sci. 172: 224-229.

Effects of elevation on N cycling in tropical forests

Scientists predict that tropical regions will receive the most dramatic increase in nitrogen (N) deposition over the next decades. This is due to increased fertilizer use, legume cultivation, fossil fuel consumption and biomass burning. There is thus a need for a better understanding of N cycling in tropical forest ecosystems. In a recent study by Arnold et al. (2009) across an Andosol (young volcanic ash soil) toposequence in Ecuador (Equitorial South America), it was revealed that gross rates of N transformations, microbial N turnover time, and δ¹⁵ N signatures in soil and leaf litter decreased with increasing elevation, indicating a decreasing N availability across the toposequence. Accompanying the above-mentioned trend was a decreasing degree of soil development with increasing elevation as indicated by declining clay content, total C, total N, effective cation exchange capacity and increasing base saturation. The study also revealed that soil N-cycling rates and δ¹⁵ N signatures were highly correlated with mean annual temperature but not with mean annual rainfall. Microbial immobilization was the largest fate of produced NH₄⁺ whereas nitrification activity was only 5-11% of gross NH₄⁺ produced. A fast reaction of NO₃^- to organic N which suggests abiotic NO₃^- immobilization, was also observed.

Reference

Arnold J, Corre MD, Veldkamp E. 2009. Soil N cycling in old-growth forests across an Andosol toposequence in Ecuador. Forest Ecology and Management 257: 2079-2087.

The importance of N:P ratio

Soil fertility in terrestrial ecosystems has received increased attention from ecologists since it is now widely recognized that nutrient availability drives ecosystem functioning and processes (Wardle and Zackrisson, 2005). N and P are believed to be the most limiting nutrients in many terrestrial ecosystems particularly forests. Availability of N and P vary considerably during soil development as P is lost through leaching and fixation while N accumulates through biological N fixation (Walker and Syers, 1976; Crews et al., 1995). Thus, young soils have the tendency to be N limited while old soils are P limited. Ecosystem studies have confirmed this relationship of N and P indicated by the N:P ratio. It has been found that the leaf N:P ratio can detect nutrient limitation for wetland terrestrial ecosystem. An N:P ratio >16 indicates P limitation which is in clear agreement with the Redfield ratio (Redfield, 1958) for marine ecosystems. An N:P ratio <14 indicates N limitation and between 14 and 16 means either N or P is limiting (Koerselman and Meuleman (1996). It has also been reported that P limitation relative to N is widespread in terrestrial ecosystems (Elser et al., 2000a and 2000b) and that it is the cause of biomass decline in forest ecosystems in strongly weathered soils (Wardle et al., 2004a). Kitayama (2005) argued, however, that despite P limitation, tropical rain forests in Southeast Asia are still able to maintain high biomass as a result of high species diversity.

Elemental stoichiometry or the ratio of key elements such as carbon (C), nitrogen (N), and phosphorus (P) in organisms is useful in analyzing how the organisms influence or is being influenced by the ecosystem in which they are found (Elser and Dobberfuhl, 1996). While the elemental stoichiometry (Redfield ratio) of 106 C: 16 N: 1P is well-established for marine ecosystems it is just starting to be applied to terrestrial ecosystems. Thus, Elser and Urabe (1999) suggested that scientists working in other ecosystems (e.g. forest) might profitably apply stoichiometric approaches to food web dynamics and nutrient cycling. This is particularly valid for terrestrial systems since autotroph biomass N:P in terrestrial and freshwater systems has been found to be closely similar (Elser et al., 2000b)

References

Crews T.E et al. 1995. Changes in soil phosphorus fractions and ecosystem dynamics across a long chronosequence in Hawaii. Ecology 76: 1407-1424; Elser, J.J., Dobberfuhl, D.R., 1996. Organism size, life history, and N:P stoichiometry. Bioscience 46, 674-685; Elser, J.J. et al. 2000a. Biological stoichiometry from genes to ecosystems. Ecology Letters 3, 540-550; Elser, J.J, et al.2000b. Nutritional constraints in terrestrial and freshwater food webs. Nature 408, 578-580; Kitayama, K., 2005. Comment on ecosystem properties and forest decline in contrasting long-term chronosequences. Science 29, 633b; Koerselman, W., Meuleman, A.F.M., 1996. The vegetation N:P ratio: a new tool to detect the nature of nutrient limitation. Journal of Applied Ecology 33, 1441-1450;Redfield, A.C., 1958. The biological control of chemical factors in the environment. Am. Sci. 46, 205-221; Wardle, D.A., Zackrisson, O., 2005. Effects of species and functional group loss on island ecosystem properties. Nature 435:806-810;Warlde, D.A et al.2004a. Ecosystem properties and forest decline in contrasting long-term chronosequences. Science 305:509-513; Wardle, D.A. et al. 2004b. Ecological linkages between aboveground and belowground biota. Science 304: 1629-1633.

Relation between N mineralization and latitude

The global distribution of soils is a function of climate and thus is related to latitude. Consequently, soil processes are known to vary with latitude. But a recent study by Jones et al. (2009) which used soils collected from 40 latitudinal points from the Arctic through to Antarctica, showed that this is not the case for key soil processes like the turnover of amino acids (amino acids represent a key pool of carbon and nitrogen in soil and their availability to plants and microorganisms is considered a major driver in regulating ecosystem functioning). They found that “soil solution amino acid concentrations were relatively similar between sites and not strongly related to latitude. In addition, when constraints of temperature and moisture were removed, they demonstrated that soils worldwide possess a similar innate capacity to rapidly mineralize amino acids. Similarly, they showed that the internal partitioning of amino acid-C into catabolic and anabolic processes is conservative in microbial communities and independent of global position. This supports the view that the conversion of high molecular weight (MW) organic matter to low MW compounds is the rate limiting step in organic matter breakdown in most ecosystems.”

Reference

Jones DL, K Kielland, FL Sinclair, R A Dahlgren, KK Newsham, JF Farrar, DV Murphy. 2009. Soil organic nitrogen mineralization across a global latitudinal gradient. Global Biogeochem. Cycles, 23, GB1016, doi:10.1029/2008GB003250