Sequestration of P fractions in the soils of an incipient ferralisation chronosequence on a humid tropical volcanic island

BACKGROUND

Phosphorus (P) is the limiting nutrient in many mature tropical forests. The ecological significance of declining P stocks as soils age is exacerbated by much of the remaining P being progressively sequestered. However, the details of how and where P is sequestered during the ageing in tropical forest soils remains unclear.

RESULTS

We examined the relationships between various forms of the Fe and Al sesquioxides and the Hedley fractions of P in soils of an incipient ferralitic chronosequence on an altitudinal series of gently sloping benches on Green Island, off the southeastern coast of Taiwan. These soils contain limited amounts of easily exchangeable P. Of the sesquioxide variables, only Fe and Al crystallinities increased significantly with bench altitude/soil age, indicating that the ferralisation trend is weak. The bulk of the soil P was in the NaOH and residual extractable fractions, and of low lability. The P fractions that correlated best with the sesquioxides were the organic components of the NaHCO₃ and NaOH extracts.

CONCLUSIONS

The amorphous sesquioxides, Fe_o and Al_o, were the forms that correlated best with the P fractions. A substantial proportion of the labile P appears to be organic and to be associated with Al_o in organic-aluminium complexes. The progression of P sequestration appears to be slightly slower than the chemical and mineralogical indicators of ferralisation.

Influences of stormwater concentration infiltration on soil nitrogen, phosphorus, TOC and their relations with enzyme activity in rain garden

Rain garden is a typical facility with many applications in urban low impact development (LID). It plays an important role in regulating runoff water quantity and quality. Two rain gardens with the discharge ratios of 20:1 and 15:1 were used as studied facilities. Seven soil sampling events were conducted from April 2017 to February 2019 to study the influences of stormwater concentration infiltration in rain gardens on soil nitrogen (N), phosphorus (P) and TOC and their relations with enzymes. The results showed that the contents of soil TN and NO₂-N + TON in gardens gradually decreased with time, while those of NH₃-N and TP increased with time. The content of NO₃-N varied greatly with time, and there was no obvious rule. TOC increased first and then decreased. Vertical distributions of N, P and TOC showed that the contents of NH₃-N, NO₂-N + TON and TN at 0-50 cm were high, so the upper soil was the sensitive area to the influence of stormwater concentration infiltration in rain gardens. The content of NH₃-N decreased gradually with the increase of soil depth, but those of NO₃-N and TP increased with the soil depth. Therefore, NO₃-N and TP migrated down with water infiltration in soil, and preventing NO₃-N and P leaching was critical for effective N and P removal though rain gardens. Soil urease (SU), sucrose (SS), protease (SP) and acid phosphatase (SAP) had a good linear relationship with N, P and TOC, and R²were all greater than 0.5.

Interplay of phosphorus doses, cyanobacterial inoculation, and elevated carbon dioxide on yield and phosphorus dynamics in cowpea

Phosphorus (P) demand is likely to increase especially in legumes to harness greater benefits of nitrogen fixation under elevated CO₂ condition. In the following study, seed yield and seed P uptake in cowpea increased by 26.8% and 20.9%, respectively, under elevated CO₂ level. With an increase in phosphorus dose up to 12 mg kg^-1, seed yield enhanced from 2.6 to 5.4 g plant^-1. P application and cyanobacterial inoculation increased the microbial activity of soil, leading to increased availability of P. Under elevated CO₂ condition, microbial activity, measured as dehydrogenase, acid phosphatase, and alkaline phosphatase activities showed stimulation. Soil available P also increased under elevated CO₂ condition and was stimulated by both P application and cyanobacterial inoculation. Higher P uptake in elevated CO₂ condition led to lower values of inorganic P in soil. Stepwise regression analysis showed that aboveground P uptake, soil available P, and alkaline phosphatase activity of soil influenced the yield while available P, and organic and inorganic P influenced the aboveground P uptake of the crop. This study revealed that under elevated CO₂ condition, P application and cyanobacterial inoculation facilitated P uptake and yield, mediated through enhanced availability of nutrients, in cowpea crop.