Bioaccessibility and bioavailability of subsoil P
Final Report Abstract
Subsoils of arable fields contain considerable amounts of phosphorus (P) in mineral form. In this project, we aimed to test whether the utilization of these subsoil resources is enhanced by the presence of biopores, which facilitate root growth into the subsoil. Specifically, we hypothesized that (i) biopores represent hotspots of P cycling in the subsoil (as indicated by the stable oxygen isotope ratio in phosphate; δ18OP), that (ii) δ18OP values are also sustained in plants, thus allowing to use them as a tracer of P uptake, that (iii) subsoil roots are able to acquire P from soil minerals such as apatite, and that (iv) the degree of P uptake from the subsoil inceases especially under water and nutrient deficiency in the topsoil. Finally, the effect of P hotspots in biopores (v) was to be implemented in root growth models, to estimate P uptake from the subsoil and its biopores. To test our hypotheses, we conducted several field, microcosm and modeling experiments, based on the development of novel methods for P-labeling and for modeling root growth in soil pores. Biopores in field experiments (i) were characterized by significantly higher contents of P in plant-available form compared to bulk subsoil, especially if the pore was inhabited by an earthworm. The analysis of δ18OP values in biopores and bulk subsoil further indicated that biopores represent a hotspot of P-cycling. This can be attributed to increased microbial activity due to root growth and earthworm activity in the pore. In a pot experiment (ii), we then aimed to also use δ18OP values in plant phosphate as a tracer of P uptake. Here, we found that δ18OP values in shoots were fully equilibrated with cell water due to the high rate of intracellular P recycling. Shoot δ18OP values thus cannot provide information on P uptake. In roots, however, up to 70% of the δ18OP value of the source P was preserved, thereby potentially providing a new analytical tool for indirect tracing of plant P uptake. These findings now require further validation. For direct tracing of P uptake by spring wheat, we conducted two rhizotron experiments with radioactive 33P-labeling. For the first one, we (iii) developed a new protocol for wet-chemical synthesis of apatite, which is suitable to be used in combination with highly labeled radioactive compounds. By adding 33P-labeled apatite to the subsoil in the rhizotrons, we could show that plants took up P from the apatite. The amount of 33P taken up and its allocation to plant compartments were controlled by the irrigation regime and were generally enhanced if the subsoil was kept moist by irrigation. Thus, plants may be able to utilize P from subsoil minerals, especially when the subsoil provides additional water resources. In the second rhizotron experiment we then (iv) varied P fertilization and irrigation in the topsoil and also added a 33P-label to the topsoil, while the subsoil contained artificial soil pores. Here, 33P was redistributed from the topsoil into subsoil roots, where it likely supported further root growth and water and nutrient acquisition. Also, we observed that root growth into the subsoil and into the subsoil pores was enhanced by fertilization and irrigation in the topsoil. Thus, contrary to our assumption, the subsoil likely does not compensate for lacking resources in the topsoil. Instead an efficient utilization of subsoil resources requires sufficient nutrient and water supply in the topsoil to support initial root growth into the subsoil. In order to include these observations into modeling, we (v) conducted a meta-analysis of root system images to identify universal patterns of root traits. Further, we developed a new model for root growth in porous media, which is now implemented in the model R-SWMS and can thus be used to develop estimates of nutrient uptake from biopores. In summary, we observed that biopores represent hotspots of P cycling. Also, plant roots growing into the bulk subsoil via biopores, are able to solubilize P from subsoil minerals such as apatite. However, root growth into subsoil biopores was stimulated by P-fertilization and irrigation, indicating that P uptake from the subsoil is additionally controlled by the supply of P and water in the topsoil. These observations can now be implemented in models of nutrient uptake and thereby promote our understanding of the effects of biopores on P acquisition from the subsoil.
Publications
- Recovering root system traits using image analysis exemplified by two-dimensional neutron radiography images of lupine.
Plant Physiology, Vol. 164. 2014, Issue 1, pp. 24-35.
Leitner, D., Felderer, B., Vontobel, P. & Schnepf, A.
(See online at https://doi.org/10.1104/pp.113.227892) - Innovative methods in soil phosphorus research: A review. Journal of Plant Nutrition and Soil Science (JPNSS), Vol. 178. 2015, Issue 1, pp. 43-88.
Kruse, J., Abraham, M., Amelung, W., Baum, C., Bol, R., et al.
(See online at https://doi.org/10.1002/jpln.201400327) - The δ18O signatures of HCl-extractable soil phosphates: Methodological challenges and evidence of the cycling of biological P in arable soil. European Journal of Soil Science, Vol. 66. 2015, Issue 6, pp. 965-972.
Amelung, W., Antar, P., Kleeberg, I., Oelmann, Y., Lücke, A., Alt, F., Lewandowski, H., Pätzold, S. & Barej, J.A.M.
(See online at https://doi.org/10.1111/ejss.12288) - A new model for root growth in soil with macropores. Plant and Soil, Vol. 415. 2017, Issue 1–2, pp. 99–116.
Landl, M., Huber, K., Schnepf, A., Vanderborght, J., Javaux, M., Bengough, A.G. & Vereecken, H.
(See online at https://doi.org/10.1007/s11104-016-3144-2) - CRootBox: A Structural-Functional Modelling Framework For Root Systems. bioRxiv: 139980
Schnepf, A., Leitner, D., Landl, M., Lobet, G., Mai, T.H., Morandage, S., Sheng, C., Zoerner, M., Vanderborght, J., Vereecken, H.
(See online at https://doi.org/10.1101/139980) - Macropore effects on phosphorus acquisition by wheat roots – a rhizotron study. Plant and Soil, Vol. 416. 2017, Issue 1–2, pp. 67–82.
Bauke, S.L., Landl, M., Koch, M., Hofmann, D., Nagel, K.A., Siebers, N., Schnepf, A. & Amelung, W.
(See online at https://doi.org/10.1007/s11104-017-3194-0) - Six months of L. terrestris L. activity in root-formed biopores increases nutrient availability, microbial biomass and enzyme activity. Applied Soil Ecology, Vol. 120. 2017, pp. 135-142.
Athmann, M., Kautz, T., Banfield, C., Bauke, S., Hoang, D.T.T., Lüsebrink, M., Pausch, J., Amelung, W., Kuzyakov, Y. & Köpke, U.
(See online at https://doi.org/10.1016/j.apsoil.2017.08.015)
