oxide (N 2 O) were short-term in nature and found emission reductions, but long-term studies are lacking, as is mechanistic understanding of reductions. Stable N isotopes have a role in elucidating biochar-N-soil dynamics. There remains a dearth of information regarding effects of biochar and soil biota on N cycling. Biochar has potential within agroecosystems to be an N input, and a mitigation agent for environmentally detrimental N losses. Future research needs to systematically understand biochar-N interactions over the long term.

Abstract. Over time scales of thousands to millions of years, and in the absence of rejuvenating disturbances that initiate primary or early secondary succession, ecosystem properties such as net primary productivity, decomposition, and rates of nutrient cycling undergo substantial declines termed ecosystem retrogression. Retrogression results from the depletion or reduction in the availability of nutrients, and can only be reversed through rejuvenating disturbance that resets the system; this differs from age-related declines in forest productivity that are driven by shorter-term depression of nutrient availability and plant ecophysiological process rates that occur during succession. Here we review and synthesize the findings from studies of long-term chronosequences that include retrogressive stages for systems spanning the boreal, temperate, and subtropical zones. Ecosystem retrogression has been described by ecologists, biogeochemists, geologists, and pedologists, each of which has developed somewhat independent conceptual frameworks; our review seeks to unify this literature in order to better understand the causes and consequences of retrogression. Studies of retrogression have improved our knowledge of how long-term pedogenic changes drive shorter-term biological processes, as well as the consequences of these changes for ecosystem development. Our synthesis also reveals that similar patterns of retrogression (involving reduced soil fertility, predictable shifts in organismic traits, and ecological processes) occur in systems with vastly different climatic regimes, geologic substrates, and vegetation types, even though the timescales and mechanisms driving retrogression may vary greatly among sites. Studies on retrogression also provide evidence that in many regions, high biomass or "climax" forests are often transient, and do not persist indefinitely in the absence of rejuvenating disturbance. Finally, our review highlights that studies on retrogressive chronosequences in contrasting regions provide unparalleled opportunities for developing general principles about the long-term feedbacks between biological communities and pedogenic processes, and how these control ecosystem development.

Soil P composition can be conveniently determined in alkaline extracts using solution "P nuclear magnetic resonance (NMR) spectroscopy, but spectral assignments are based on fragmentary literature reports of model compounds in various extraction matrices. We report solution "P 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 L57 ppm, phosphatidyl ethanolamine L75 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 "P NMR spectroscopy of soil extracts be performed at 20°C.COIL P EXISTS in a multitude of chemical forms, which L.3 differ widely in their behavior in the soil environment. Information on soil P composition is a fundamental prerequisite to understanding nutrient and organic matter dynamics in both natural and managed systems. However, such information remains limited. Organic P forms are particularly enigmatic, and a large proportion remains unidentified in most soils (Harrison, 1987). Many such compounds prove difficult to extract chemically, but can nonetheless provide a source of P for plant uptake (e.g., Gahoonia and Nielsen, 1992;Chen et al., 2002).Detailed information on soil P composition can be obtained by alkaline extraction and solution 31P NMR spectroscopy. This procedure was first used by Newman and Tate (1980) to investigate P in New Zealand grassland soils, since when it has become the method of choice for determining soil P composition (e.g., Hawkes et al.,

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Long-term changes in soil phosphorus influence ecosystem development and lead to a decline in the productivity of forests in undisturbed landscapes. Much of the soil phosphorus occurs in a series of organic compounds that differ in their availability to organisms, but changes in the relative abundance of these compounds during pedogenesis remain unknown. We used alkaline extraction and solution phosphorus-31 nuclear magnetic resonance spectroscopy to assess the chemical nature of soil organic phosphorus along a 120,000-year postglacial chronosequence at Franz Josef, New Zealand. Inositol phosphates, DNA, phospholipids, and phosphonates accumulated rapidly during the first 500 years of soil development characterized by nitrogen limitation of biological productivity, but then declined slowly to low concentrations in older soils characterized by intense phosphorus limitation. However, the relative contribution of the various compounds to the total organic phosphorus varied along the sequence in dramatic and surprising ways. The proportion of inositol hexakisphosphate, conventionally considered to be relatively recalcitrant in the environment, declined markedly in older soils, apparently due to a corresponding decline in amorphous metal oxides, which weather to crystalline forms during pedogenesis. In contrast, the proportion of DNA, considered relatively bioavailable in soil, increased continually throughout the sequence, due apparently to incorporation within organic structures that provide protection from biological attack. The changes in soil organic phosphorus coincided with marked shifts in plant and microbial communities, suggesting that differences in the forms and bioavailability of soil organic phosphorus have ecological significance. Overall, the results strengthen our understanding of phosphorus transformations during pedogenesis and provide important insight into factors regulating the composition of soil organic phosphorus.

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Information on the composition and dynamics of soil phosphorus (P) remains limited, but is integral to understanding soil biogeochemical cycles. We used solution 31P nuclear magnetic resonance (NMR) spectroscopy to characterise NaOH-EDTA extractable P in 29 permanent pasture soils from England and Wales (total carbon 29-80 g kg-1 soil, clay 219-681 g kg-1 soil, pH 4.4-6.8). Total P ranged between 376 and 1981 mg P kg-1 soil, of which between 45 and 88% was extracted with NaOH-EDTA. The extracts were dominated by orthophosphate monoesters (29-60% extracted P) and inorganic orthophosphate (21-55% extracted P), with smaller concentrations of orthophosphate diesters (2-10% extracted P), pyrophosphate (1-7% extracted P), phosphonates (0-3% extracted P), and traces of polyphosphates. Orthophosphate diesters were subclassified into phospholipids (1-7% extracted P) and DNA (1-6% extracted P). Signals slightly downfield of inorganic orthophosphate were tentatively assigned to aromatic orthophosphate diesters similar in structure to R-(-)-1,1'-binaphthy1-2,2'-diy1 hydrogen phosphate. Such signals are rarely detected in soil extracts, but were present in relatively large concentrations in the samples analysed here (2-5% extracted P). Relationships between functional P groups and soil properties suggested that the various functional groups are involved in the soil P cycle to different extents. In particular, concentrations of orthophosphate monoesters appeared to be controlled by the potential for chemical stabilisation in soil, whereas DNA and pyrophosphate were strongly correlated with the microbial biomass, suggesting an active involvement in biological nutrient turnover.