Microbial mineralization of phosphorus in soil was determined by Jinshui et al., 2006. It

is likely that available P (using Olsen-P as the indication) could make a major contribution to

the initial immobilization of P by microbial biomass in this soil. The amounts of inorganic P

in Al-P and Fe-P fractions in this soil significantly decreased, by a total value of 12 mg P kg–1

soil, which was 2.6 times larger than the increase in biomass P (BP). The majority of P

released from Al-P and Fe-P fractions in highly weathered subtropical soil is likely converted

into organic fractions. The P assimilated by microbial biomass in soil amended with rice

straw was unable to be quantified as from native soil available P or P released from the straw,

because it was not labeled with 32P or 33P. Increasing the size of the microbial biomass by

organic amendment enhances P assimilation, but the turnover of BP behaves in different

patterns as biomass C (BC) in highly weathered subtropical soil.

They found such type of result from subtropical soil through an incubation period.

After the end of 43-day incubation, they observed the only significant change in the soil

without organic amendment (CK) was in biomass P (BP) (p < 0.01; Fig. 2).

Fig. 2 Changes in soil microbial biomass P in the subtropical soil following amendments with

glucose and rice straw.

Because the amounts of Olsen-P and total P in Fe-, Al-, and Ca-bound, and occluded

fractions changed little, as compared to the values determined at the beginning (Fig. 3; Table

1), BP decreased in this soil was presumably transformed into organic fractions.

Fig. 3 Changes in soil Olsen-P in the subtropical soil following amendments with glucose and

rice straw.

Throughout the 43-day incubation period, the amount of BP increased (4–5 mg kg–1 soil; Fig.

2) in the soil amended with glucose (G2) was very close to the decrease in Olsen-P (3–4 mg

kg–1 soil; Fig. 3). It is likely that available P (using Olsen-P as the indication) could make a

major contribution to the initial immobilization of P by microbial biomass in this soil. By the

end of their incubation, the amounts of inorganic P in Al-P and Fe-P fractions in this soil

significantly decreased, by a total value of 12 mg P kg–1 soil (p < 0.01; Table 1), which was

2.6 times larger than the increase in BP. This is in accord with previous suggestion that

organic amendment enhances microbial activities to release P from Fe and Al oxides (He and

Zhu, 1998; Frossard et al., 2000; Chen and He, 2002). The majority of P released from Al-P

and Fe-P fractions in highly weathered subtropical soil is likely converted into organic

fractions.

In contrast to the case with glucose amendment (G2), no significant change in Olsen-P was

observed in soil amended with rice straw (both of RS2 and RS4), though the amounts of BP

increased by 7.3 mg P kg–1 soil (p < 0.01) by 7 days for RS4. This suggests that P released

from rice straw (containing 0.128% P) used in their study can compensate microbial

assimilation of soil available P (Olsen-P), as P in plant residues can be released rapidly and

effectively used by soil microorganisms (Kouno et al., 2002; Kwabiah et al., 2003). In their

study, the P assimilated by microbial biomass (e.g., 7.3 mg P kg–1 soil for RS4) in soil

amended with rice straw was unable to be quantified as from native soil available P or P

released from the straw, because it was not labeled with 32P or 33P.

After 43-day incubation, the amount of Fe-P in the soil amended with rice straw at 4 mg kg –1

soil (RS4) significantly decreased (Table 1). Furthermore, the total amount of P released from

Fe-P fraction (5.1 mg P kg–1 soil) and supplied with rice straw (12.8 mg P kg–1 soil) was

remarkably (about 10 mg P kg–1 soil) larger than the increase in BP (7.7 mg P kg–1 soil over

the initial value; Fig. 1) in this soil. These results implicate that straw amendment at this rate,

like glucose amendment, may also be able to enhance the release and organic transformation

of P bound with Fe oxides in highly weathered subtropical soil.

Oehl et al. (2004) find out basal organic phosphorus mineralization. They found organic

farming affected the size, structure and diversity of the soil microbial community, the amount

and quality of substrates available for basal mineralization, and through changes in soil

structure, the habitat of microorganisms and the related access to substrates. A part of the

mineralized P could be increasingly masked by the release of 33PO4 from organic compounds

synthesized after labelling the soil. Most of the 33PO4 was taken up within the first three days

after soil labelling, and thereafter, 33PO4 incorporation increased slightly until 60 days. This

shows that a significant 33PO4 incorporation into the microorganisms occurred, and that, in

spite of the fact that no net release was observed, P exchange between the soil

microorganisms and the soil solution did take place. The 33PO4 release from Po compounds

that were labelled through microbial 33PO4 immobilization and that were subsequently re-

mineralized could have resulted in an underestimation of basal soil Po mineralization in non-

irradiated soils. The most probable reason for greater P mineralization in γ-irradiated than

non-irradiated soils is the release of mineralizable Po compounds from decaying microbial

cells.They found such type of result under different farming system.

Soil microbial biomass C, N and P, phosphatase activity, soil respiration and, in turn, C

mineralization, were higher in bio-dynamic (DYN) than bio-organic (ORG) and lowest in

conventional-mineral (MIN) soil (Tables 3), supporting previous results obtained in the same

field experiment (Oberson et al., 1996; Fliessbach and Mader, 1998; Mader et al., 2002).The

recording of the time course of soil respiration during incubation is an important prerequisite

for the measurement of basal Po mineralization (Oehl et al., 2001b).

Table 3

Microbial P released by chloroform treatment of the soils Pchl and daily C mineralization

(Cmin) during the phase of basal soil respiration

Mean of n replicates; sem: standard error of the mean; within a characteristic, different letters

show significant differences between the investigated soils (Duncan’s multiple range test).

Provided the temperature and moisture are controlled during the incubation experiment, basal

respiration is a function of: (1) abiotic factors such as pH and clay content that affects

microbial biomass and community structure (Anderson, 1994; Shannon et al., 2002); (2)

amount and quality of organic matter input and native SOM (Wander et al., 1994); (3) the

degree of SOM protection in stable soil aggregates (Breland and Eltun, 1999; Shepherd et al.,

2002). Organic farming affected the size, structure and diversity of the soil microbial

community, the amount and quality of substrates available for basal mineralization, and

through changes in soil structure, the habitat of microorganisms and the related access to

substrates (Fliessbach et al., 2000).

As well as constant basal respiration, which was attained after two weeks of pre-incubation,

constant isotopic exchange parameters are a prerequisite for the determination of basal Po

mineralization (Oehl et al., 2001b).

This shows that basal Po mineralization is a significant process in delivering available Pi to

the soil solution, but clearly at a lower rate than physicochemical processes. The Pi

concentration in the soil solution remained constant during the mineralization experiment.

Therefore, abiotic or biotic processes consumed as much orthophosphate as had been

produced by basal Po mineralization. The daily Po mineralization rates were similar to those

found for several Northern American Mollisols (0.9–4.2 mg P kg21 day21, Lopez-Hernandez

et al., 1998). Organic P mineralization decreased in the order DYN > ORG ≥ MIN (Table 5),

i.e. in the same order as other soil microbiological properties (Tables 3), but differences in Po

mineralization rates between ORG and MIN soils were small and non-significant. The

significance of Po mineralization resulting from flush effects and re-mineralization after the

addition of organic substrates should be investigated in soils characterized by different soil

microbial activity.

Table 5

Daily P mineralization rates in non-irradiated (Pmin) and in g-irradiated soils (g-Pmin)

determined 7 and 10 days after starting the mineralization experiment, respectively;

isotopically exchangeable P during 24 h (modE24 h) and the ratio between the daily P

mineralization and modE24 h (Pmin(7d):E24 h)

A part of the mineralized P could be increasingly masked by the release of 33PO4 from

organic compounds synthesized after labelling the soil. In incubation study carried out under

the same experimental conditions as the presented study, i.e. at constant basal respiration and

without significant net changes in the size of the soil microbial biomass, 33PO4 incorporation

into microbial Pchl was between 3% (MIN) and 9% (DYN) of the applied 33PO4 (Oehl et al.,

2001a).

The 33PO4 release from Po compounds that were labelled through microbial 33PO4

immobilization and that were subsequently re-mineralized could have resulted in an

underestimation of basal soil Po mineralization in non-irradiated soils. This effect can be

estimated from the turnover of microbial P at steady state, i.e. if the size of microbial P

remains unchanged (Oehl et al., 2001a; Oberson and Joner, 2004). The most probable reason

for greater P mineralization in γ-irradiated than non-irradiated soils is the release of

mineralizable Po compounds from decaying microbial cells (Seeling and Zasoski, 1993). This

signifies that Po mineralization rates obtained on the γ-irradiated soils represent an

overestimated biochemical mineralization. Finally, γ-irradiation increases extractable Mn and

can reduce extractable Fe contents (Wolf et al., 1989; Trevors, 1996).

The C and Po mineralization rates obtained in our study therefore suggest that basal Po and C

mineralization are not necessarily closely linked, in contrast to the mineralization of freshly

added organic material (Dalal, 1979; Gressel and McColl, 1997).

Another experiment was done by Dossa et al. ( ) to determine the phosphorus

mineralization potential of semiarid Sahelian soils amended with native shrub residues.

In the arid and semiarid Sudano Sahelian zones, soils are inherently of low fertility (Bationo

and Buerkert, 2001), and intensive cropping combined with shorter fallow periods and greater

livestock pressure is causing significant loss of organic matter and depletion of nutrient

reserves in soils (Sanchez et al., 1997).

In Senegal, there are two dominant, native shrubs, Piliostigma reticulatum (DC.) Hochst and

Guiera senegalensis J. F. Gmel., which potentially can provide more organic inputs to

cropped fields than any other source in the Sahel (Lufafa et al., 2008).

P. reticulatum was higher in soils beneath the shrub canopy than in soils from outside the

canopy after the incubation period. In the G. senegalensis soils, apart from manure-amended

soils, all treatments had net negative P release relative to the control (E. L. Dossa et al.,

2008).

E. L. Dossa et al.,’s study was similar to that reported by Kramer and Green (1999) in a

juniper microsite study, but is in contrast to other studies where higher P contents have been

reported in soils beneath woody species canopies than outside the canopy.

E. L. Dossa et al., found such type of result by determining the phosphorus mineralization

potential of semiarid Sahelian soils amended with native shrub residues.

The P mineralization study was conducted on the semiarid Sahelian soils and shrub residue

treatments as the P mineralization study according to the (Stanford and Smith, 1972) method

with a slight modification.

For both species, leaf material released high levels of P at time zero (Table 3) which was in a

similar range of the total P released by shrub residues over the incubation period. Manure-

amended soils had the highest rate of P mineralization throughout the 118 day of incubation

(E. L. Dossa et al., 2008).

Table 3

Initial P leached from soils amended with the different organic residues

In P. reticulatum soils, leaf-amended soils released the smallest amount of P in soil beneath

the canopy (Fig. 3A) and leaf+stem released slightly (but not significantly) more P than the

control during the first 62 days of incubation. On soils outside the P. reticulatum canopy

amended with residues, similar amounts of P were released to that of the control (Fig. 3B).

Cumulative P at the end of the incubation period for P. reticulatum was higher in soils

beneath the shrub canopy than in soils from outside the canopy(E. L. Dossa et al., 2008).

In the G. senegalensis soils, apart from manure-amended soils, all treatments had net negative

P release relative to the control (Fig. 3C and D). The leaf+stem amendment tended to release

less P than leaves only for this species; however, these differences were not significant. In

contrast to the trend seen with P. reticulatum-residue treatments, soils from outside the G.

senegalensis canopy released more P than soils from beneath shrub canopy (Dossa et al.,

2008).

Fig. 3. Cumulative P leached from soils beneath (A) and outside shrub canopy (B) in P.

reticulatum residue-amended soils; and beneath (C) and outside (D) shrub canopy in G.

senegalensis residue–amended soils.

For both P. reticulatum and G. senegalensis soils, manure amended soils had the highest net

P release. The greater P release from soils collected outside the canopy of G. senegalensis

than that from soils beneath the canopy was not expected (Kwabiah et al., 2003).

The greater release of P was found with P. reticulatum litter amended to soil beneath the tree

canopy than outside. This difference in findings may be due to difference between the soils

that develops beneath G. Senegalensis vs. C. pinnata (E. L. Dossa et al., 2008).

P measured in solution during this incubation experiment does not reflect the true net P

mineralization during litter microbial decomposition. Rather, it represents the P released that

is in excess of what is sorbed on surfaces of soil minerals or biologically immobilized

(Sharpley and Smith, 1989; Iyamuremye et al., 2000).

The various shrub materials did not increase release of P over the unamended control

suggesting these materials would have limited potential to supply the immediate nutrient

needs of the crops. It is possible that shrubs grown in fields under higher fertilizer regimes for

summer crops could have high P contents which could increase P release from these residues

during decomposition (Whitford and Kay, 1999).

Phosphorus mineralization in soil aggregates after long-term tillage and cropping was

determined by Alan L. Wright in 2008. He said that land management practices that increase

aggregation and organic matter levels may also increase P retention and stability.

Long-term P fertilization and soil subsidence is the export of P from the Everglades

Agricultural Area (EAA) through canal systems into Everglades’s wetlands, which has been

implicated in causing deterioration of water quality and alterations to the natural ecosystem

(Childers et al., 2003). Phosphorus in organic pools indicates long-term sequestration

potential under flooded conditions, but may be unstable under drained conditions due to soil

oxidation and organic P mineralization (Sanchez and Porter, 1994). Whereas P sequestered in

inorganic P pools is stable under drained conditions, it may be unstable upon flooding and

conversion to seasonally flooded prairies since flooding can stimulate the release of mineral-

bound P (McGrath et al., 2001).

P stability may increase with increasing aggregation. Phosphorus in inorganic pools adsorbed

or precipitated with Ca, Fe, and Al is stable when soils are maintained in the same condition

leading to P fixation, but may be susceptible to dissolution and regeneration upon change in

land use (Ivanoff et al., 1998). The distribution of P within chemically defined pools, such as

labile, inorganic, and organic fractions, provides an indication of the potential stability of P in

soil, and may be different between land uses. (Wright and Reddy, 2007).

Tillage influenced soil chemical properties and the distribution of P in labile and recalcitrant

fractions. The Ca-bound fraction was the chemical fraction most affected by land use and was

significantly greater for sugarcane (244 kg P ha-1) than pasture (65 kg P ha-1. Elevated Ca

levels enhanced precipitation and adsorption of P from fertilizer or mineralized from organic

matter, resulting in higher P in Ca-bound fractions for sugarcane than pasture (Sanchez and

Porter, 1994; Snyder, 2005).

The differences in P content of inorganic fractions between land uses likely reflects P inputs

by fertilization and higher organic matter turnover rates for sugarcane cropping (Castillo and

Wright, 2008). Soils that have a higher proportion of their total P in organic forms would be

less prone to release P which would minimize eutrophication of proximal aquatic systems.

Flooding has the opposite effect for mineral-associated P (Sanchez and Porter, 1994), as

dissolution from inorganic pools can increase after flooding (Graham et al., 2005).

Pasture soils tended to have a higher proportion of macroaggregates than sugarcane which

contributed to greater P storage in organic than inorganic pools. Tillage and cultivation

decreased organic P, and the incorporation of CaCO3 into soils by tillage enhanced P

sequestration in the Ca-bound fraction. Phosphorus accumulation in inorganic pools may be

unstable and ultimately result in regeneration of P upon onset of flooded conditions that occur

during high rainfall events or conversion to the seasonally flooded prairie ecosystem. Since

most of the P in pasture soil was in organic pools, flooding of these soils would decrease

organic matter decomposition and increase P stability. The conversion of current land uses to

seasonally flooded prairies may have a more dramatic effect on P release from sugarcane than

pasture soils since soils under sugarcane have more P in inorganic pools.