Pasture degradation decreases organic P content of tropical soils due to soil structural decline
Introduction
Highly weathered soils of the humid tropics typically have low total and available phosphorus (P) contents and P is often the primary limiting nutrient to plant growth (Nziguheba and Bünemann, 2005, Vitousek et al., 2010). On deforested areas in South America, pastures sown with introduced grasses (Brachiaria spp.) represent a dominant land use, and the majority of pastures exist in some stage of degradation (Jimenez and Lal, 2006). Pasture degradation is understood as a marked reduction in livestock production due to a significant decrease in plant biomass production and invasion of non-palatable plant species, and it leads to appearance of bare soil patches, soil compaction and reduced soil microbial biomass (Boddey et al., 2004, de Oliveira et al., 2004). Pasture degradation has enormous economic implications. For example, in Brazil every year about 8 million ha of degraded pastures require considerable investment for renewal and/or recovery (Jank et al., 2014), with estimated costs of 100 to 200 US$ ha− 1, i.e., around 1 billion US$ in total (FAO, 2006). Soils of degraded pastures have been shown to store less carbon (C) than soils of productive pastures (Asner et al., 2004, Fonte et al., 2014). Pasture over-grazing and reduced pools of available nitrogen (N) and P are seen as the principal causes of degradation (Dias-Filho et al., 2001, Fisher et al., 2007). While the importance of N cycling and availability has been demonstrated previously (Boddey et al., 2004), a mechanistic understanding of the role of P in maintaining pasture productivity is missing.
Because most inorganic P is strongly sorbed, organic P has been suggested to play a critical role in sustaining P availability in highly weathered soils (Tiessen et al., 1984, Turner et al., 2006). Biological P cycling through plant uptake, residue decomposition and microbial turnover, which includes synthesis of organic P, appears to be crucial to protect P from sorption and to maintain P availability in highly weathered tropical soils (Oberson et al., 2006). Organic P constitutes 16% to 65% of total P in highly weathered soils (Nziguheba and Bünemann, 2005).
A single-step procedure involving sodium hydroxide and EDTA (ethylenediaminetetraacetate) is often used for the extraction of soil organic P from strongly weathered tropical soil (Turner, 2008). Additions of enzymes to various soil extracts have been used to characterize hydrolyzable forms of organic P, as reviewed by Bünemann (2008), e.g., addition of acid phosphatase or phytase to soil water extracts (Fox and Comerford, 1992, Shand and Smith, 1997). As different enzymes hydrolyze different organic P bonds, this approach delivers information on the chemical form of organic P in soils (He and Honeycutt, 2001, Turner et al., 2002a). Based on P release by acid phosphatase, phytase and nuclease from defined organic P compounds, Annaheim et al. (2013) classified the hydrolyzed P into simple monoesters, myo-inositol hexakisphosphate and nucleic acids. Keller et al. (2012) used the same approach to classify organic P in NaOH–EDTH extracts. Enzyme-hydrolyzable organic P identified by phytase addition to EDTA extracts was shown to be greater in Oxisols under no till than with conventional tillage (Pavinato et al., 2010), suggesting that enzyme-hydrolyzable organic P is a sensitive indicator for the impacts of soil management on soil P status.
Soil aggregation is considered important for regulating the storage and turnover of soil organic C (Six et al., 2000a, Six et al., 2000b) and nitrogen (N) (Bosshard et al., 2008). Relatively few studies have examined the effect of aggregation on P and these have suggested that P availability, P concentrations and forms can be influenced by aggregation (McDowell et al., 2007, Wang et al., 2001). For example, P uptake was greater in plants grown in large aggregates (2–6 mm) than in plants grown in small aggregates (< 0.5 mm) of highly weathered Oxisols because P bound to large aggregates was more readily desorbed (Wang et al., 2001). The water extractable P concentration decreased with smaller aggregate sizes in cropped soils in Canada (Messiga et al., 2011). The concentrations of orthophosphate, monoesters, diesters and pyrophosphates determined by 31P-nuclear magnetic resonance (31P-NMR) spectroscopy increased with decreasing aggregate size in a soil from New Zealand, while phosphonates and polyphosphates were unaffected (McDowell et al., 2007). We also note that the effect of management (or landuse) on P concentrations in different structural components is amplified when the proportion of these components on a whole soil mass basis is also affected.
In a recent study, the distribution of soil mass among aggregate size classes was found to differ between degraded and productive pasture soils (Fonte et al., 2014). Although soils of both pasture types had a high aggregate stability, the proportion of large macroaggregates (> 2000 μm) was significantly higher in productive (65 g 100 g− 1) than in degraded pasture soils (56 g 100 g− 1). Soil of productive pastures had 20% higher total C and N contents (in g kg− 1 soil) than degraded pastures. These differences in total soil organic matter (SOM) between pasture types were largely explained by a greater C content in the large macroaggregate fraction, and more specifically in the microaggregates (53–250 μm) occluded within this macroaggregate fraction (Mmicros). Interestingly, there was no difference in total P content between pasture types, but organic P content was found to be nearly 40% greater in soils of productive vs. degraded pastures. The findings of Fonte et al. (2014) suggest that different organic P contents in the bulk soils of productive and degraded pastures could be related to differences in soil structure and the distribution of C across aggregate fractions. Specifically, these results lead us to hypothesize that greater organic P content is associated with the greater protection of SOM in the Mmicros fraction of productive pastures.
To further elucidate the role of aggregation in the P status of highly weathered tropical soils we examined the distribution of organic, inorganic and available P across aggregates and occluded macroaggregate fractions of the same degraded and productive pasture soils studied by Fonte et al. (2014). To obtain information on organic P forms, we determined enzyme-hydrolyzable organic P classes by enzyme additions. Finally, we studied the relationship between C and organic P across the different soil structural components.
Section snippets
Site description, experimental design and soil sampling
The study was conducted on nine farms located in the deforested Amazon region of Colombia. All farms are situated within a 30 km radius of the city of Florencia, in the Department of Caquetá (1°36′50″N 75°36′46″W) with an average elevation of 280 m.a.s.l. The region has a humid tropical climate with a mean annual precipitation of 3400 mm and a mean annual temperature of 25 °C. The mildly undulating topography is characterized by acid soils, mainly Oxisols and Ultisols (Mosquera et al., 2012) with
Phosphorus concentrations in bulk soil, aggregates and fractions
Concentrations of total and available P in the bulk soil highlight the low P status of the soils, with average total P concentrations of 362 mg P kg− 1 and less than 7 mg kg− 1 of available P extracted using anion exchange resins (Table 2). Organic P concentration was significantly (37%) higher in the bulk soil of productive vs. degraded pasture soils, while total and available P concentrations were similar. Organic P constituted on average 19% of total P in degraded and 26% in productive pasture
Aggregation affects organic and inorganic P contents
Our results suggest that soil aggregation is crucial in maintaining organic P contents in tropical grassland soils. In particular, most organic P was stored within large macroaggregates, and more specifically within microaggregates occluded within macroaggregates (Mmicros, Fig. 3.4). Since the concentrations of enzyme-hydrolyzable organic P largely changed in parallel with organic P across aggregates and occluded fractions, the Mmicros fraction was also an important site for the storage of
Conclusions
This study shows a clear linkage between soil structure and organic P. Large macroaggregates, and in particular the microaggregates occluded within macroaggregates, were identified as an important site of organic P storage. Soils under degraded pastures contained fewer large macroaggregates in which C and organic P can be physically protected. The observed reduction of organic P, in turn, affected all identified enzyme-hydrolyzable P forms as well as enzyme-stable P.
Degraded and productive
Acknowledgments
We would like to thank all the participating farmers for the support and permission to conduct research on their land. Special thanks go to Katherine Herrera Vanegas and Miller Gomez Mosquera for their friendship, hospitality, and extensive assistance in the field. We express our gratitude to Gonzalo Borerro and others at CIAT who facilitated laboratory activities there. Finally, we thank Arne Korsbak from Novozyme (DSM Nutritional Products) for the supply of Ronozyme NP (M). This research was
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Current address: Department of Plant Sciences, University of California, Davis, USA.