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Abstract Band 73

Bauke, S . (2018): Phosphorus acquisition from arable subsoils 

Abstract

Phosphorus (P) is one of the major limiting nutrients in agricultural crop production, even though large quantities of P are present in soils and especially in the subsoil. As plant access to subsoil nutrients likely depends on overall resource availability and penetration resistance, I therefore evaluated whether and to which extent the utilisation of these subsoil P resources can be modulated by fertilisation and the presence of biopores. In detail, I tested (i) the effect of long-term P and nitrogen (N) fertilisation on P stocks and P cycling in the subsoil, (ii) the effect of short-term earthworm activity on P pools in biopores and (iii) whether biopores increase P availability and P cycling in the subsoil. Further, I (iv) tested a combined effect of fertilisation and soil pores on P acquisition from the subsoil. Since this test could only be conducted under highly artificial conditions, I finally (v) tested the application of oxygen (O) isotopes in plant phosphates as a tracer for plant P uptake.

I sampled (i) two long-term fertilisation trials with NPK, NK and PK fertilisation treatments on a loamy-clay soil in Gießen and on a sandy soil in Thyrow to a depth of 100 cm. Further, in a field trial on a loamy soil in Klein-Altendorf with conventional fertilisation, I twice sampled bulk soil and biopore linings. One sample set (ii) consisted of subsoil biopore samples with a known recent history with or without earthworm occupation, the other sample set (iii) included the complete soil profile to 105 cm depth in biopores with unknown history. Finally, I conducted two pot experiments to trace P uptake by roots. For this purpose, spring wheat was grown (iv) in rhizotrons filled with subsoil and a P-depleted topsoil in treatments with or without irrigation, with or without P-fertilisation and with or without macropores. To test the feasibility of P tracing by oxygen isotopes (v) pre-grown spring wheat was transferred to hydroponic solution or soil with unlabelled fertiliser and 18O-labelled water for one week.

P stocks in pools of varying chemical extractability and P speciation were determined by Hedley sequential extraction as well as X-ray absorption near-edge structure (XANES) spectroscopy. The concentration of plant-available P was additionally determined by calcium-acetate calcium-lactate (CAL P) extraction. For indications on P cycling in soils, I analysed the O isotopic composition of phosphate extracted by 1M HCl (δ18OHCl P). Oxygen isotopic compositions in plant phosphates were determined after extraction by 0.3M trichloro-acetic acid (TCA, δ18OTCA P). Uptake of radioactive 33P was quantified by radio-imaging and liquid-scintillation counting (LSC).

I found that (i) subsoil P stocks increased as a function of fertilisation regime in the order NK < NPK < PK independent of the soil type, although this trend was generally more pronounced at the sandy site in Thyrow than at the loamy-clay site in Gießen. Also, in Thyrow NK fertilisation resulted in subsoil δ18OHCl P values further from equilibrium than in the NPK treatments, indicating a lower degree of P cycling. The other treatments had only a limited effect on δ18OHCl P values. When excluding fertilisation effects, I found that (ii) short-term earthworm activity can significantly increase the concentration of CAL P and labile P pools in subsoil biopores. However, (iii) labile P is precipitated to more stable HCl-extractable bonding forms, as evident in δ18OHCl P values closer to equilibrium in biopores than in bulk soil. Additional supplies of plant-available P in biopores are thus apparently in excess of crop P uptake in biopores. Also, in the rhizotrons (iv) shoot and root biomass as well as root growth into the subsoil and 33P uptake were promoted by fertilisation and irrigation and so was 33P uptake. However, again these processes were not significantly affected by pores in the subsoil. The alternative application of O isotopes as a tracer is complicated by the fact that (v) phosphate in shoots was quickly equilibrated with the labelled water, i.e. the value of the P fertiliser was lost. However, up to 70% of the fertiliser δ18O values was preserved in the roots, which might thus provide a promising tool for tracing P acquisition in field studies.

In summary, fertilisation as well as biopores affect P cycling in the subsoil. However, the contribution of biopores was limited to P cycling in biopore linings, while the presence of pores in the subsoil hardly improved P acquisition. Both field and rhizotron studies indicated that efficient utilisation of P resources from the subsoil should not aim at compensating nutrient deficiency in the topsoil, but is achieved best when P and other factors such as N and water are supplied in sufficient amounts in the topsoil.

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