Soil P cycle

Most of the P in uncultivated soils is concentrated at or near the soil surface. Inorganic orthophosphate r unlike 

Additions of organic matter to soil as plant, animal and microbial residues provide the energy needed to sustain the cycling of nutrients in the soil, as well as returning P to the soil for recycling. The mechanisms by which the P enters soil are consequently of considerable importance, and, as already 

The extremely complex interrelationships that exist in the rhizosphere between detrital processing, mineralisation of organically-bound nutrients and nutrient uptake, makes field investigation of these processes difficult. Consequently, an alternative approach has been to use sterilised soil and added consumer and decomposer organisms (microcosms) in various combinations in the laboratory to investigate the role of the microfauna in the soil P cycle. 

In the example shown (Fig. 5), rhizospheres were simulated using combinations of bacterial, amoebal and nematode 

FIGURE 5. Effects of amoebae on levels of NaHCOa-P and microbial P over a 24 day incubation in soil microcosms. From Cole et al..  

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This review is an attempt to bring together the current knowledge on the soil P cycle, emphasising particularly the rates and pathways of P through soil organic matter, and the availability of P to plants. 

Cole et al. chose a natural ecosystem for their simulation modelling studies of the soil P cycle. 

Nazaries L, Murrell JC, Millard P, Baggs L, Singh BK. Methane, microbes and models Fundamental understanding of the soil methane cycle for future predictions. Environ. Microbiol. 2013 15 2395-2417. 

P cycling, pre chain emissions, animal welfare, economics, biodiversity, product quality, soil quality, and landscape aesthetics [60]. Whole farm model (WFM) uses pasture growth and cow metabolism for predicting CH4 emissions in dairy farms. Also included in the WFM is climate and management information. However, recent reports also suggests that WFMs may incorrectly estimate CH4 emission levels as they do not take into account the DMI and diet composition while predicting the enteric CH4 emission. This low prediction efficiency of WFMs may lead to substantial error in GHG inventories [10,11],... 

Another possibility for plants to influence the P cycle is the hydraulic redistribution of water. This is the redistribution of water from wet to dry soil areas via the roots, which has been suggested to have an impact on the availability of P due to better mobility of inorganic P in wet soil (Lambers et al. 2006). McCulley et al. (2004) found that the concentration of extractable P was greater at depth than in the top meter of the soil in several arid and semi-arid systems in the southwestern USA and that nutrients were uplifted from this depth. They proposed that hydraulic redistribution of water from the soil surface to depths up to 10 m by roots was the mechanism by which P and other nutrients were mobilized and could be taken up by plants. 

Microbial biomass P Chloroform fumigation extraction Indicate biologically active fraction of soil phosphorous control P cycle dependent upon organic amendments Brookes et al. (1982)... 

Oberg G, Sanden P (2005) Retention of Chloride in Soil and Cycling of Organic Matter-Bound Chlorine. Hydrol Process 19 2123... 

Figure 6.7. Simplifed soil carbon cycling scheme. Major inputs (plant litter) to and outputs (respiration and erosion) from the soil carbon reservoir. The observed flux of C out of the soil can be modeled by assuming three pools of carbon an active pool with a turnover time on the order of years, an intermediate pool with a turnover time on the order of decades to centuries, and a passive pool with a turnover time on the order of millennia. The decomposition constant is k = 1/t. Subscripts a, i, and p refer to the active, intermediate, and passive C pools, respectively. Adapted with permission from Amundson, R. (2001). The carbon budget in soils. Annu. Rev. Earth Planet. Sci. 29, 535-562.

The microbial regulation of the P cycle in Forest ecosystems is tightly coupled to soil development and the change of phosphorus pools from the predominance of primary... 

Phosphorus cycling in soils. The cycling of P in soils (see Fig. 1) has received much attention, both in terms of fertilization and the natural development of ecosystems. Of the approximately 122,600 Tg P within the soil/biota system on the continents, nearly 98% is held in soils in a variety of forms. The exchange of P between biota and soils is relatively rapid, with an average residence time of 13 years, whereas the average residence time of... 

Significant advances have been made in our understanding of the dynamics of the global P cycle. Interestingly, many of these advancements are derived from the widespread application of P geochemical techniques developed for soils and sediments over the last several decades, but only recently adopted by oceanographers. These techniques have allowed us to further elucidate the dynamics of P transformations during... 

The rhizosphere soil of Genista aetnensis (Biv.) DC. hosts a microbial population that is responsible for biological P cycling. This may be considered to be the final stage of a strategy adopted by brooms to preserve P, the most limiting nutritive element of this soil. This strategy is probably what makes the broom plants able to colonize the inhospitable soils on the flanks of Mount Etna. 

Figure 6.5 shows short-term P cycle (Kimura 1989). Most of P is present in geosphere and hydrosphere, but it is small in amount in the atmosphere. Thus, the fluxes between atmosphere and geosphere and atmosphere and hydrosphere are small (Fig. 6.6). The amount of P in soil is large ((96-160) x lO g). This is due to an increase in P flux as fertilizer. The amount of P in biosphere is 26 x 10 " g and (5-12) X 10 g in terrestrial and marine organisms, respectively. Figure 6.6 shows six reservoir models for the global biogeochemical cycle of P.

According to Pierrou, about 13 Mt P reach the soils of the world annually by each of these pathways, while a similar amount (ca. 12 Mt) is lost through soil erosion and leaching. Plants have been estimated to remove somewhere between 178 and 240 Mt P, while biological returns amount to about 100 Mt annually. An even faster water-based P cycle, with turnover times measured in months rather than years as for the land-based cycle, is essentially closed, with the very small quantity of P entering the oceans in rivers being approximately balanced by that removed in sediments. ... 

A consideration of the P cycle in soil on a pedogenetic time scale of several thousand years is a useful starting point in attempting to answer these questions. 

Both these methods, which were mainly developed using near neutral to alkaline soils, provide an opportunity of more clearly defining the role of the microbial biomass in the P cycle. They now need to be tested on a wider range of soil types, including acid soils, to confirm their value for general use. 

Research on P cycling in grassland soilsstimulated interest in methods for separating soil P fractions according to their activity in the P cycleand led to the development of a fractionation scheme (Fig. 3) based on the use of different soil extractants. Four P pools were identified in this scheme as follows ... 

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