Phosphorus cycling in grasslands soils
Most P in terrestrial ecosystems is present in soil. Total P in soils ranges from 200 to 800 mg kg-1 in older/highly weathered and younger/less developed soils.(Khaledian et al., 2018). Grazing systems have impacts on nutrient cycling promoting nutrient flux modifications, where animals are catalysts, and soil is the central compartment, in which nutrient cycling processes occur (Costa et al., 2014). Organic matter inputs result in very uneven nutrient availability, consequently, in vegetation dynamics. In fertilized and natural pastures ecosystems the preferential localization of roots in upper soil layers is due to nutrient accumulation in surface horizons as a consequence of the absence of ploughing and low P ion mobility (Jobbágy and Jackson, 2001). The P cycling is controlled simultaneously by geochemical and biological processes (Fig. 1) being determined in extent by the complex nature of interactions between inorganic, organic and microbial forms of P (Condron and Newman, 2011). Thus, soil P is derived from different organic and inorganic sources added to soil by human activity or derived from soil inherent processes.
Plant available phosphorus in grassland soils and its uptake by plants
P uptake by plant roots occurs mainly as phosphate ions (H2PO4- and HPO42-) from the soil solution, which depends on release of mineral phosphate through solubilization and release of organic P through mineralization (Hinsinger, 2001). During phosphate uptake, P-depleted area is formed close to roots, since phosphate uptake is faster than its diffusion in soil (Bhat and Nye, 1974). Plants mediates positive and negative interactions via root exudations by rhizosphere (Fig. 3), which is defined as the soil volume around living roots influenced by root activity and their ability to secrete a wide range of compounds into rhizosphere, being one of the remarkable metabolic features of plant roots. The release of inorganic P through solubilization or mineralization may be influenced by the plants through rhizosphere processes (Richardson et al., 2009).
The P uptake by grassland plants may vary between fertilizer formulation, composition and application, soil characteristics, plant species, and planting-harvesting schemes. Chemical reactions and microbial activity affect P availability for plant uptake. Under acid conditions, P is held tightly by Al and Fe in soil minerals (Borie and Rubio, 2003; Escudey et al., 2001; Hinsinger, 2001; Redel et al., 2016; Velásquez et al., 2016a). Under alkaline conditions, P is held tightly by soil calcium (Abbasi et al., 2015; Divito and Sadras, 2014; Waldrip et al., 2015). The P is strongly adsorbed onto soil colloids and does not form volatile compounds, so its cycle takes place exclusively in the biosphere and losses of P by leaching are generally small.
Management practices to improve soil phosphorus availability
The P bioavailability in productive grasslands is affected by several factors of managements including soils nutrients interactions (Paredes et al., 2011). Due to high phosphate fertilizer prices following phosphate rock (RP) future scarcity (Cordell et al., 2009; Reijnders, 2014), there is a research need for strategies by which P fertilizers can be used more effectively and improve their uptake efficiency by plants. Possible options to do this include the management of: pasture plant species (Cougnon et al., 2018; Klabi et al., 2018; Maltais-Landry, 2015), grazing intensity (Baron et al., 2001; Chaneton et al., 1996; Mundy et al., 2003; Neff et al., 2005; Simpson et al., 2012), cutting intensity, stocking density (Alfaro et al., 2009; Gao et al., 2016; Simpson et al., 2015), and animal manure inputs (Abdala et al., 2015; Adeli et al., 2003; Costa et al., 2014; Duan et al., 2011; Geisseler et al., 2011; Mclaughlin et al., 2004; Takeda et al., 2009; Vanden Nest et al., 2016). In grazed pasture, improving the recycling of P may be achieved through grazing management that reaches more uniform animal excreta distribution, hence, P returns to the pasture (Syers et al., 2008; Williams and Haynes, 1990).
Mowing, grazing and stocking density
Mowing trials are used to measure pasture responses to nutrients because they are much cheaper to operate than grazing trials, but in mowing trials the influence of the grazing animal through excreta return, treading and defoliation is absent (Morton and Roberts, 2001). Long-term biomass removal contribute to a less labile organic P relative to SOC attributed to enhanced mineralization of labile organic P in response to continued depletion of soil inorganic P (Boitt et al., 2017). Simpson et al. (2012) found that inorganic and organic P in the readily available fraction and the residual P were higher in soil from clippings left treatments compared with the no mowing and clippings removed treatments. Moreover, the P uptake for the clipping left was 51-54% higher than no mowing treatment.
Grazing can improve soil quality over time, maintaining higher moderate and labile P pools and contributing increases on pasture yield (Costa et al., 2014). Increasing animal densities reduces the selection for palatable vegetation patches within a grazing camp, and that this can reduce the spatial heterogeneity in vegetation vigor over time (Venter et al., 2019). Also, differences in the stage of the plant before start the grazing is important to consider as nutritive value. Lawson et al. (2017), demonstrated that 3-leaf stage in tall fescue appears to be the most productive of the grazing-management plant persistence and the ability of lactating dairy cows to consume the dry matter (DM) grown efficiently.
Livestock overgrazing is one of the most important factors that results in pasture degradation (Cavagnaro et al., 2018). Livestock grazing alters the cycles of soil nutrients in pastures ecosystems by the interactions between plants and the soil, maintaining species diversity by light or moderate grazing, while opposite using heavily grazing (Costa et al., 2014; Li et al., 2011). By ingesting herbage, grazing animals encourage pasture plants to grow and therefore take up more nutrients from the soil (Williams and Haynes, 1990). Simpson et al. (2015) found that the stock carrying capacity of pasture growing on P deficient soil is increased by increasing the total soluble P concentration of the soil, which could be attributed to grazing promotes root exudation feeding the microorganisms on the rhizosphere, increasing their activity and consequently the P cycle efficiency (Costa et al., 2014). However, Da Silva et al. (2014) didn’t found an interaction of grazing intensity and cattle dung input in a field experiment with Italian ryegrass under four different grazing intensities (0.10, 0.20, 0.30 or 0.40 cm), indicating that high soil P were associated with high grazing intensities.
Alfaro et al. (2009) establish that in grazed areas, depending on the stocking rate and the grazing efficiency, the amounts of P recycled can reach levels equivalent to the amounts applied as mineral fertilizers, especially in low input systems. However, Gao et al. (2016) found that P content was unaffected by stocking rate (2.4 and 4.8 animal unit months ha-1), grazing treatment (continuous vs. discontinued) and their interactions in a cattle grazing study that started in 1949 until 2013. Grazing intensity could promote root proliferation depending of plant species, for example for smooth bromegrass (Bromus inermis Leyss) greater at medium grazing than either heavy or light grazing pressure whereas for meadow bromegrass (Bromus biebersteinii Roem. & Schult.) was greatest under light grazing pressure (Mapfumo et al., 2002).
Chemical characterization of poultry manure compost and soil
Chemical analyses of PM and soil samples were carried out according to the methodology described by Sadzawka et al. (2006). pH was determined in H2O with a 1:2.5 PM sample: water solution ratio. Total C and nitrogen (N) were determined by dry combustion using a CHN auto-analyzer (CHN NA 1500, Carlo Erba). No carbonate was present in the soil; therefore, soil C is considered to be exclusively organic. Basic exchangeable cations (Ca, Mg, Na and K) were extracted with 1 M ammonium acetate (pH 7) and determined by atomic absorption spectrophotometry (AAS). Determination of Al and Fe present in the -humus complex (Alpyro and Fepyro) were determined by extraction with 0.1 M sodium pyrophosphate diphosphate (pH 10) and amorphous Al and Fe (Alox and Feox) were determined by extraction with 0.2 M ammonium oxalate pH 3.0 (van Reeuwijk, 2002). Al and Fe in pyrophosphate and oxalate extracts were measured by atomic absorption (UNICAM, 969 AA spectrometer) at 309 and 248 nm, respectively.
Phosphorus concentration and fractionation
Total P of PM and soil was determined in extracts by alkaline digestion with sodium hypobromite (NaBrO) (Dick and Tabatabai, 1977). Plant available P was extracted with sodium bicarbonate (0.5 M NaHCO3 at pH 8.50) (Olsen and Sommers, 1982).
The nature of P in PM and soil was determined by sequential extraction using a scheme based on that proposed by Hedley et al. (1982). Briefly, to quantify readily available P, 1 g of sample was extracted with 25 mL of deionized H2O. The samples were shaken during 16 h and then centrifuged at 5000g for 20 min. The soil solution was filtered and stored at 4 °C. The remaining sample was extracted, as described above, with sodium bicarbonate (0.5 M NaHCO3 at pH 8.5), followed by sodium hydroxide (0.1 M NaOH) and hydrochloric acid (1 M HCl) to extract P of decreasing lability. In all extracts, inorganic P was measured directly by colorimetry (Murphy and Riley, 1962), total P was measured by NaBrO digestion (Dick and Tabatabai, 1977), and organic P was calculated as the difference between total and inorganic P.
Table of contents :
Thesis summary and outline
Table of contents
Chapter I. General introduction
1.1 General introduction
1.2 Phosphorus status of pasture ecosystems
1.3 Phosphorus cycling in pasture soils
1.3.1. Soil phosphorus forms
1.3.2. Phosphorus inputs and outputs
1.4. Plant available phosphorus in pasture soils and its uptake by plants
1.5. Managements practices to improve soil phosphorus availability
1.5.1. Mowing, grazing, and stocking density
1.5.2. Pasture plant species
1.5.3. Animal manure input as fertilization management
1.6. Conclusion and perspectives
CHAPTER II. “Soil available P on southern Chilean pastures under composted poultry manure is regulated by soil organic carbon, and iron and aluminum complexes”
2.2 Material and methods
2.2.1 Study farms and soil sampling
2.2.2. Chemical characterization of poultry manure compost and soil
2.2.3. Phosphorus concentration and fractionation
2.2.4. Soil particle size distribution
2.2.5. Statistical analysis
2.3.1. Chemical characterization of PM added to pasture soils
2.3.2. Soil chemical characterization and soil particle size distribution on Andisols under pastures amended with PM
2.3.3. Soil phosphorus distribution of Andisols under pastures amended with PM
2.3.4. Relationship between soil parameters and soil particle size
CHAPTER III. “Synergistic and Antagonistic effect of poultry manure and phosphate rock on soil P availability, ryegrass production, and P uptake”
3.2 Materials and methods
3.2.2. Growth chamber experiment
3.2.3. Soil analyses
3.2.4. Biomass analyses
3.2.5. Synergistic and antagonistic effect of mixture
3.2.6. Statistical analysis
3.3.1. Total soil C, N and P concentration
3.3.2. Soil phosphorus forms
3.3.3. Microbial biomass P
3.3.4. Shoot and root biomass production
3.3.5. Shoot and root concentrations and uptake
3.3.6. Relationship between soil parameters and plant parameters
3.3.7. Synergistic and antagonistic effect between PM and RP on soil plant parameters
3.4.1. Impact of organic and inorganic P amendments on C, N, and P stoichiometry and microbial biomass P
3.4.2. Impact of organic and inorganic P amendments on nutrient uptake and biomass production and soil P forms
3.4.3. Synergistic and antagonistic effect of the combined application of PM and RP
CHAPTER IV. “Impact of poultry manure and rock phosphate amendment on C allocation in the rhizosphere of ryegrass plants”
4.2 Materials and Methods
4.2.2. Growth chamber experiment
4.2.3. Microbial biomass C
4.2.4. Soil organic matter density fractionation
4.2.5. Sources of soil organic carbon
4.2.6. Statistical analysis
4.3.1. Microbial biomass C
4.3.2. Plant-derived C, poultry manure compost- derived carbon, and native rhizosphere soil C
4.3.3. Total C and N derived from plant and poultry manure compost input and their distribution in SOM fractions
4.4.4. Relationship between parameters
4.4.1. Effect of amendments on soil C and N status
4.4.2. Effect of amendments on C transfer from plant to soil
Chapter V. “General discussion and concluding remarks”
5.1 General discussion
5.2 Concluding remarks and future directions
Appendix 1. Publications and presentations
Appendix 2. Laboratory incubation experiment
Appendix 3. Relationship between soil and plant parameters from control and soil amended with poultry manure compost, phosphate rock and their combination
Appendix 4. Nutrient concentration in shoots and roots