Distribution of P, C and Abuscular Mycorhiza in agricultural soil profiles after long-term contrasting liming and P fertilization

Project: Research

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Phosphorus (P) is an essential nutrient for plant growth, which is taken up by plants as phosphate. Phosphorus is unevenly distributed and relatively immobile in soils. The amount of plant available P in unfertilized soil is often inadequate to meet crop requirements. Thus, P is applied to agricultural fields as inorganic or organic fertilizers. The amount of fertilizer applied often have to exceed the uptake by plants until a certain level of P in soil is obtained where soil P availability does not limit crop production which results to build-up of P in the soil (Jianbo et al., 2011). In many agricultural systems in which the application of P to the soil is necessary to ensure plant productivity, the recovery of newly applied P by crop plants in a growing season is very low because more than 80% of P is retained in the soil in organic and inorganic forms (Holford, 1997). In many regions in the USA, EU and Denmark, especially areas with intensive animal production, P accumulation in soils have accelerated to levels exceeding the crop requirements with consequent increased potential for P losses to the aquatic environment (Kronvang et al., 2009). The rock phosphate, which is the raw materials used for producing P fertilizers and feed phosphates, is a non-renewable and finite resource. The depletion of P reserves is expected to occur in 50–400 years depending on P supply and demand dynamics (Reijnders, 2014; Sattari et al., 2012; Scholz and Wellmer, 2013). Although, recent assessment of global accessible P reserve by US Geological Survey revealed more than four fold increased in phosphate rock reserve with Morocco having the largest reserve of global P (Elser & Bennett, 2011). But the cost of mining of phosphate rock and the loss of P through leaching and erosion call for a concious efforts by scientists, policy makers and other stakeholders involving in P mining and utilization to find ways of ensuring sustainable P use. Therefore, sustainable long-term P management strategies are important to ascertain optimal utilization of P stored in soils and P added with fertilizers and organic amendments.

There have been many studies of the movement of P through soil surface processes on agricultural land (McDowell et al., 2001); whereas processes that enhance P losses and P redistribution in the soil profile via subsurface pathways are not as well understood. Under circumstances favouring preferential flow in soil macro-pores subsurface pathways play an important role in P loss from agriculture increasing the risk of surface water eutrophication ((Heckrath et al., 1995); Sims et al., 1998; Turner and Haygarth, 2000). Increases in P concentration in subsoils following land application of fertilizer or animal manure are indicative of translocation of applied P to the subsoil (Rubæk et al., 2013). Subsoils have most often the capacity to sorb P from water leaching through the profile due to its high content of Fe, Al or Ca (Sims et al. 1998) if the water leaching through the soil is in close contact with the soil matrix. Subsoil will therefore typically reduce P leaching from the entire profile compared to the leaching observed from topsoil alone (Liu et al. (2012). Phosphorus enrichment in subsoils will on the long term increase the P leaching risk associated with non-preferential flow too. This is especially relevant for highly fertilized sandy soil profiles poor in the main P sorbents Fe and Al oxides and rich in P. Here the critical level of degree of phosphorus saturation (DPS) may be exceeded and such soils are therefore vulnerable to P leaching via matrix flow (Leinweber et al., 1997). Thus, increase in DPS through sustained surplus P additions reduce the capacity of top and subsoils to retain P (Nair and Graetz, 2002). What is lacking is quantitative estimates of the extent of downward transfer of P in the soil profile and better understanding their specific controls. Lack of models to predict the movement of P in the soil profiles is posing challenge, as leaching of P is often not described in details.

Rubaek et al. (2013) proposed four processes that could be responsible for P transfer to and from subsoil: (1) leaching of dissolved and colloidal P from top soil to subsoils, (2) deeper soil tillage resulting in the partial mixing of top and subsoil, (3) tillage erosion where topsoil is removed from upslope convex areas and deposited on downslope concave areas and (4) bioturbation by soil living animals and redistribution due to root activity. The fourth point, regarding bioturbation, includes processes related to growth of Plant roots, soil fauna and microbial turnover including the activities of Arbuscular Mycorrhiza Fungi (AMF). However, very few studies deal with the bioturbation processes in relation to the distribution and redistribution of P in soil profiles. A net transfer of P from subsoil to topsoil through P uptake by plant roots from subsoil may take place. Rubaek et al. (2013) found that the total P content in the 0.25-0.50 m layer of the more clayey soil declined 38 mg P kg-1 between 1987 and 1998, corresponding to a transfer out of this layer, presumably removed with the crop, of 13 mg P kg-1 y-1.

The sorption and desorption characteristics of soil determines the availability of applied P in soil solution for plant uptake. When soil pH is increased, P adsorption is expected to decrease because of increase in hydroxyl competition for the adsorption sites and an increase in negative surface charge, whereas an increase in pH with presence of exchangeable Al can lead to formation of Al hydrous oxides with high surface area and high affinity for P (Anjos et al., 1987). Therefore, the effect of increase in soil pH on P adsorption is largely depends on the balance between hydroxyl competition for adsorbing sites and the formation of new adsorbing surfaces in the presence of Al. Majumdar et al. (2005) investigated the effect of liming on P adsorption and desorption behaviour of acidic Alfisols and Entisols of Meghalaya, India, they concluded that P supply parameters interms of quantity, intensity and buffering capacity of soil increased with increasing in P level and decreased with lime application in both Alfisol and Entisol, and concluded that interaction of lime and P addition was significant in supply parameters. Also, Satos and Comerford (2005) after conducting an experiment on influence of pH on inorganic P adsorption/desorption, reported that liming the soil had the double effect of reducing P sorption (up to 4.5 kg ha-1 of remaining P in solution) and enhancing P desorption (up to 2.7 kg ha-1 of additionally released P into solution).

The PLEASE model have been developed to determine P losses by leaching on a field scale. This model is called Phosphorus LEAching from Soils to the Environment (PLEASE) and it is based on the rate of P release and lateral water flux from soils to surface waters (Schoumans et al., 2013).
In the past, phosphate saturation degree of soil is used as an indicator of potential risk without given any information on actual P loss to groundwater or streams. (Schoumans et al., 2013)compared the PLEASE model with other common methods for estimating potential P losses based on soil P tests or phosphate saturation degree in a study in the Netherlands. They reported a weak correlation between these indicators and the calculated P losses for these other methods which do not take into consideration the differences in local hydrology. Hence, they concluded that PLEASE model proves to be a promising method for indicating differences in P loss by leaching between fields within a catchment.

Arbuscular Mycorrhiza Fungi (AMF) are integral components of root systems in agricultural plants that provides the plant mineral nutrients as e.g. P and increase plant tolerance to abiotic stress conditions. In exchange, photosynthetic compounds are transferred to the fungus. AMF abundance in soil is reduced by P fertilization, mechanical disturbances (e.g. tillage) or changes of host plants in crop rotation system. Application of P reduces the percentage roots which are colonized (% RLC) by AMF and the reduction depends on the type or species of AMF (Veiga et al., 2011). High P fertilization may on the long term decrease the presence and richness of soil AM communities (Johnson, 1993). Smith et al. (1992), reported that percent colonization is not a valid measure of fungal biomass per plant, and decreases with increasing soil P can be due to increase in root length, with constant AM-biomass per plant. According to Smith and Smith (2011), true suppression of fungal biomass per plant and decreased frequency of arbuscules may occur only at very high soil P, thus, it cannot be inferred that a plant (or indeed a fungus) is in control of the symbiosis simply on the basis of changes in percent colonization. Thompson et al. (1986) reported that both colonized root length and intensity of AMF biomass per colonized root length decreased at high P levels. The actual cause of the suppression of colonization is still unresolved, but involves shoot-to-root signaling, as shown by supply of P only to foliage or in split pots (Sanders, 1975; Balzergue et al., 2011). According to Graham and Abbott (2000), growth depression may be linked to high carbon demands of the fungi that are not offset by increased nutrient uptake. Inoculum potential of AMF in different layers, potential of AMF population in relation to P transport and changes in functions of AMF with different treatments, need to be critically examined. The influence of lime application on AMF colonization and the relationship between AMF diversity and soil chemical properties had been elucidated by Guo et al. (2012) in a long-term liming experiment in Australia in which they reported higher intensity and abundance of the mycorrhizal colonization in lime treatment than unlimed treatments.
The concentration of soil organic carbon (SOC) is expected to increase in the soil profiles following the application of fertilizer or organic manure. Bai et al. (2013) reported that after 15 year of fertilization, significantly higher SOC accumulation was observed under the two combined inorganic P plus manure treatments probably due to the activity of soil organisms including plant roots which caused soil mixing and distribution of SOC to deeper layers of the soil profiles. The increase in soil pH as a result of liming improves the soil conditions for plant growth, thereby increase plant productivity resulting in larger organic matter inputs inform of dying roots and decaying crop residues with consequent increase in SOC stocks. The higher calcium and ionic strength of soil solution resulting from lime application, enhance the flocculation of clay minerals with formation of stable aggregates, and thus the increase in soil structure, thereby contributing to improvement of SOC physical protection (Paradelo et al., 2015). This may also reduce the microbial mineralization of SOC (Paradelo et al., 2015). However, increase in soil pH enhance microbial activity thereby increase organic matter mineralization with consequent decrease in SOC. All the processes enumerated above indicate that liming acid soil may have net positive effects on SOC stocks.

As described in the sections above, long-term liming and P fertilisation strategies of agricultural soils may have profound effects not only on soil pH and soil phosphorus content and crop yield, but also on other characteristics such as P sorption characteristics, the AMF community and the soil organic matter content. Improving our understanding of this is crucial for maintenance of soil fertility on the long term

The core of this project is linked to the long-term field experiment on liming and P fertilization at St. Jyndevad field station, where treatments with different levels of P fertilization and liming have been maintained since 1944. Archived data and archived soil samples will be included and combined with new samplings, analyses and experiments based on soil from these experiments. This will be supported by a study from long-term fertilisation field experiment at Askov.
Effective start/end date01/08/201631/07/2019

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