SUMMARY
Studies of nutrient cycling in forests span more than 100 yr. In earlier years, most attention was given to the measurement of the pools of nutrients in plants and soil and of the return of nutrients from plant to soil in litterfall. The past 20 yr or so have seen a major concentration on the processes of nutrient cycling, with particular emphasis on those processes by which the supply of nutrients to the growing forest is sustained. In the more highly productive forests, up to 10 tonnes of litter of low nutritional quality is deposited annually on the forest floor. The decomposition of this litter, the mineralization of the nutrients it holds, and the uptake of nutrients by tree roots in the carbon-rich environment which results are the themes of this review.

Studies of decomposition of litter in forests have been dominated by the role of nitrogen as a limiting factor, a domination which reflects the preponderance of studies of temperate forests in the Northern Hemisphere. For many forests of the world growing on soils of considerable age, it seems more probable that growth and nutrient cycling are limited by phosphorus (or some other element). There is increasing evidence for a number of forests that phosphorus is immobilized in the first stages of decomposition to a significantly greater extent than is nitrogen. Advances in research will depend, as with studies of soil organic matter, in denning and developing analytical techniques for studying biologically active forms of potentially limiting nutrients, rather than total elemental concentrations.

The availability of phosphorus in forests is sustained by phosphorus cycling. More than 50% of the total phosphorus in the surface soils is in organic forms and much of the more labile phosphorus is in the form of diesters. Phosphorus availability is determined by competition between biological and geochemical sinks, and it is clear that the sinks in the rhizosphere (plant roots, microorganisms, soil mineral and organic components) are extensively modified by active processes (e.g. production of exudates, nutrient storage in a variety of organic or polymeric forms and nutrient transport away from sites of uptake). There is abundant evidence that roots of many species exude compounds which have the ability to solubilize sources of phosphorus of otherwise low availability. The significance of root exudates (for example, phosphatases, organic acids) in the functioning of perennial ecosystems has yet to be quantified and there are conflicting reports as to the effects of simple organic acids on phosphorus availability. The distribution of phosphorus sinks and their relative competitiveness and their modification are topics of fundamental importance for future research.

In contrast to the mineralization of phosphorus, our knowledge of transformations and availability of nitrogen in forest soils is well-developed. Net nitrogen mineralization rates approximate rates of nitrogen return in litterfall but the contribution of nitrification is variable. Nitrification is not inhibited by the low pH of many forest soils and there is increasing evidence of nitrate immobilization by microorganisms and of increased diversity and better competitiveness for NH4+ of nitrifying microorganisms than has previously been accepted. Variability in rates of nitrification is often interpreted as being due to allelopathy. Hypotheses invoking allelopathy are more or less untestable, and it seems likely that new techniques using 15N in situ will lead to a more fundamental understanding of nitrogen transformations in forest soils. Recent studies in coniferous forest soils have highlighted the short (< 1 d) turnover time of NH4+.

Finally, it seems that forest soils are resistant to major changes in patterns of nitrogen mineralization (and certainly, because of the large number of sinks, in patterns of phosphorus mineralization) following disturbance by natural events such as wind-throw and fire, and by man-made events such as logging and fertilizing. The long-term disturbance by acid rain is a more complex matter since forest ecosystems are not adequate buffers for nitrate.

Nutrient cycling in forests.

Phosphorus (P) is essential for plants and animals because of its role in vital life processes, such as in photosynthesis in plants and energy transformations in all forms of life. It also has a significant role in sustaining and building up soil fertility, particularly under intensive systems of agriculture.

Chemistry of Phosphorus Transformations in Soil

Soil P composition can be conveniently determined in alkaline
extracts using solution 31P nuclear magnetic resonance (NMR) spectroscopy,
but spectral assignments are based on fragmentary literature
reports of model compounds in various extraction matrices. We report
solution 31P NMR chemical shifts of model P compounds, including inorganic
phosphates, orthophosphate monoesters and diesters, phosphonates,
and organic polyphosphates, determined in a standardized soil
P extractant (0.25 M NaOH and 0.05 M EDTA). Signals from nucleic
acids (DNA –0.37 ppm, RNA 0.54 ppm) and phospholipids (phosphatidyl
choline 0.78 ppm, phosphatidyl serine 1.57 ppm, phosphatidyl ethanolamine
1.75 ppm) could be differentiated in the orthophosphate diester
region, and were identified in a sample of cultured soil bacteria. Inorganic
and organic polyphosphates could be differentiated by the presence
of a signal at –9 ppm from the a phosphate of organic polyphosphates.
Some orthophosphate diesters, notably RNA and phosphatidyl
choline, degraded rapidly to orthophosphate monoesters in NaOH-EDTA
although DNA, other phospholipids, and orthophosphate monoesters
were more stable. Changes in probe temperature had a marked
influence on signal intensities and the relative magnitude of signals
from orthophosphate monoesters and inorganic orthophosphate, and
we suggest that solution 31P NMR spectroscopy of soil extracts be
performed at 20°C.

/pdf/phosphorus-31-nuclear-magnetic-resonance-spectral-1b6ssyhvzn.pdf

Phosphorus-31 nuclear magnetic resonance spectral assignments of phosphorus compounds in soil NaOH–EDTA extracts

Chemistry and dynamics of soil organic phosphorus.

https://repository.si.edu/bitstream/handle/10088/3976/Turner_Cade_Condron_and_Newman_2005.pdf?sequence=1

Extraction of soil organic phosphorus.

Phosphorus compounds in concentrated extracts of a range of highly-weathered Karri forest soils were identified and quantified by 31 P-NMR. The extracts were prepared using a chelating, cation-exchange resin in preference to NaOH extraction as the former proved more suitable for use with NMR analysis. Concentrations of alkali- or resin-extractable P in the soils of this study were about an order of magnitude less than in soils which have been used in previous 31 P-NMR studies of soil-P. As forest age increased, the proportion of extracted-P present in forms other than inorganic orthophosphate also increased. Monoester-P was the largest fraction of the extracted organic-P followed by diesters, pyrophosphate and traces of polyphosphates and phosphonates. Sugar phosphates (e.g. glucose-6-phosphate or ribose-5-phosphate), as well as inositol-P and choline-P, resonate at frequencies similar to those measured for soil monoesters. Buffered cation exchange resins may be useful in future studies of highly-weathered soils.

L.T. Byrne

Papers

31P-NMR analysis of phosphorus compounds in extracts of surface soils from selected karri (Eucalyptus diversicolor F. Muell.) forests