Mineralisation

The process of decomposing organically bound nitrogen (e.g. amino sugars, proteins, nucleic acids, and urea) to NH3 and NH4+ is known as N mineralisation. Due to the biological nature of decomposition, temperature and moisture strongly influence the rate at which mineralising bacteria, fungi and protozoa operate. Whilst climatic factors are the primary regulators of NH3 production, the quantity and quality of the decomposing substrate, soil fauna, soil aeration and texture, C:N ratios, and pH are also important (Curtin, et al., 1998; Leiros, et al., 1999; Gonzalez & Seastedt, 2001; Booth, et al., 2005). NH3 and NH4+ produced by

mineralisation are made available for assimilation by plants or microbes but can also be released back to the atmosphere (through volatilisation), abiotically fixed within the soil matrix, or transformed via alternate microbial pathways such as autotrophic nitrification or anaerobic ammonium oxidation (Figure 2-2). Therefore, low soil NH4+ concentrations do not necessarily reflect low rates of mineralisation if immobilisation and consumption processes are substantial. Where the net mineralisation rate is determined by measuring NH4+

accumulation after consumption, it often seriously underestimates the total amount of

mineralisation taking place (i.e. the gross mineralisation rate) (Davidson, et al., 1990; Hart, et al., 1994; Neill, et al., 1999). In recent years, the limitation of net mineralisation as an

estimate of N turnover has led to a shift towards measurements of gross mineralisation in the literature; the method most commonly employed being the isotope pool dilution technique (Kirkham & Bartholomew, 1954). For gross mineralisation, this approach involves enriching the soil with ¹⁵NH4+ and then observing dilution of the atomic % enrichment over time as ¹⁴N is mineralised and added to the total NH4+ pool. Gross consumption of NH4+ is also estimated

22 Table 2-1: Summary of gross mineralisation and nitrification rates in tropical soils.

Location Land Cover Elevation

Panama¹ a. Old-growth lowland forest b. Montane forest Ecuador¹ a. Old-growth lowland forest

b. Montane forest Australia^‡ a. Old-growth lowland forest

b. Montane forest

Costa Rica a. Old growth lowland forest

b. Forestry plantations 40 25.8 ~4000 4.5

Puerto Rico a. Lower montane forest b. Palm forest

b. Albizia plantation 450-550 21 4600

10-14^†

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by observing the change in the size of the NH4+ pool over time. The number of studies recording rates of gross mineralisation is still relatively few when compared with estimates of net mineralisation, and Table 2-1 summarises those publications reporting gross rates within the lowland tropics. Some montane (i.e. >1000m a.s.l.) sites are also included where rates at higher elevations are compared with lowland N transformations. For the most part,

researchers have used the isotope pool dilution technique to derive process rate estimates.

However, the barometric process separation (BaPS) technique has also been used to estimate gross nitrification rates in Australia (Breuer, et al., 2002; Kiese, et al., 2002; Kiese, et al., 2008). Although experimental difference such as incubation time, sampling depth and preservation make comparison difficult, some broad generalisations are possible: Firstly, rates of mineralisation in old-growth forests are highly variable with seasonal averages

ranging from 0.3 to 8.34 g N m^-2 d^-1 in Brazilian Oxisols (Neill, et al., 1999; Doff Sotta, et al., 2008). Rates in Hawaiian leguminous tree plantations were even higher (up to 16 mg N kg^-1 d^-1), although no figures for bulk density are available to convert these rates to an aerial basis (Garcia-Montiel & Binkley, 1998). Temporal variability in N mineralisation within old-growth tropical forests is likely to be less variable than in temperate climax communities, where low temperatures restrict microbial activity during the winter months. The strong control that temperature has over mineralisation is illustrated by the fact that rates are often, but not always, inversely related to altitude (Arnold, et al., 2009; Corre, et al., 2010).

Decomposition usually increases with precipitation, however, very high rainfall can depress microbial activity and restrict oxygen diffusion into soils having a negative effect on rates of mineralisation (Holtgrieve, et al., 2006).

Some lowland forests experience “hot moments” of mineralisation activity when land is cleared and burned prior to agricultural use or when soils are wetted following a period of

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drought (McClain, et al., 2003; Kiese, et al., 2008). Generally, the latter event occurs in environments that have a pronounced dry season although, even within the wet tropics, extremely high rates of mineralising activity may result with the onset of rain following periods of drier weather (Luizão, et al., 1992; Davidson, et al., 1993; Wong & Nortcliff, 1995;

Eaton, et al., 2011; Luizão, et al., 1989). Temporal variability to mineralisation rates has been reported during land preparation for oil palm re-planting. Specifically, rates of net

mineralisation increased following cutting, chipping and windrowing of mature palms into plantation soils prior to replanting (Khalid, et al., 1999). Spatial variability within oil palm plantations may also occur through, for example, the control of undergrowth vegetation close to the palm trunk where root density is greatest, or re-incorporation of empty fruit bunches and pruned palm fronds in plantation inter-rows. Specifically, rates of net mineralisation in a mature Amazonian plantation increased with distance from the palm trunk, which the authors attributed to increased microbial activity, the presence of undergrowth vegetation and

decreased bulk density in the plantation inter-rows (Schroth, et al., 2000). High root density within a 1m radius surrounding the palm trunk also resulted in decreased soil NO3

-concentrations and NO3- leaching relative to plantation inter-rows, thereby highlighting the effect of management practices on the spatial variability of nitrate (Schroth, et al., 2000). In Costa Rica, Matson et al. (1987) report an approximate 3-4 fold increase in gross

mineralisation rates immediately after forest clearance and burning, however, rates returned to pre-disturbance levels within six months. Commercial forestry plantations have been

responsible for an increase in gross mineralisation in sub-tropical Australian hoop pine plantations (Burton, et al., 2007), and a decrease in Costa Rican Cordia alliodora (Spanish elm) plantations (Silver, et al., 2005). The increased rates in hoop pine were attributed to recent disturbance and higher soil temperatures in the plantations relative to forests (Burton,

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et al., 2007), whereas mineralisation rates declined in the Spanish elm plantations concurrent with a decline in microbial biomass (Silver, et al., 2005). Where pastures are established, any initial increase in mineralisation rates after forest clearance is often followed by a decline in mineralisation to rates below those of the original forest as pastures age and organic matter returns decline (Reiners, et al., 1994; Neill, et al., 1999). Conversely, replacement of montane forest with N-rich cattle pasture grass in Ecuador increased gross mineralisation rates

concurrent with an improvement in litter quality and an increase in the number of fungi and gram-negative bacteria (Potthast, et al., 2012). The magnitude of change in mineralisation rates following land-use change is likely, therefore, to depend on the change to climatic factors such as increased temperatures or higher soil moisture through canopy and root biomass removal, the initial soil N status, and the nature of disruption to soil microbial populations and C and N storage. These parameters in turn are affected by alterations to soil physical properties such as compaction, aeration, and disturbance level and frequency.