Abstract:
Addition of organic waste such as sewage sludge (biosolids) has been
recognised as one of the cost effective method of waste recycling practice of
maintaining levels of organic matter in agriculture. Biosolids are a slow-release
nutrient (N and P) which also improves soil physical and microbiological
properties and as such offer a promising plant nutrient with less impact on soil
than mineral fertilisers. Implementation of wastewater treatment directive
91/271//EC has caused large amount of sewage sludge production and
availability. Hence, management of biosolids to provide available phosphorus
through soil recycling would reduce reliance on finite rock phosphates (over 85%
P-fertilisers) extraction, protect or minimise environmental problems such as
eutrophication and also help the resource-limited farmers particularly in
developing countries, to mitigate P-fertiliser limitation in the soil. More awareness
of the impact of different agricultural fertiliser management practices on soil
quality and sustainability has led to more interest of combining organic residuals
with inorganic fertilisers to prevent further fertility decline and degradation of soil.
Amongst recent nutrient integration, was the nutrient-balanced sludge-based
(biosolids with urea and potash) called organomineral fertiliser (OMF), which
shows agronomic efficiency of phosphorus management when applied to the
field crops such as winter wheat.
However, to manage phosphorus mineralisation effectively in the organomineral
fertiliser treated soil, it is important to understand the effects of urea components
in OMF during mineralisation of phosphorus in soil. This research aimed to
understand the influence that the urea component of an organomineral fertiliser
(OMF) has during phosphorus mineralisation in soil. Therefore urea granules
were grounded into powdered form and mixed with grounded biosolids pellets
as a source of phosphorus to obtain organomineral fertilisers.
This whole research included two different control soil incubation experiments
(1a & 1b) both observed over a 60 days period. Incubation experiment 1a
(involved mixing various rates of biosolids and urea in soil) had two different soil
samples from the grassland (sandy clay loam) and arable (clay loam) sites. In terms of initial soil phosphorus content, the grassland and arable soils are
classified as P-index 1 and 2 respectively. Soil samples were analysed for pH,
mineralisable nitrogen (NH4 +
, NO3 -
), available phosphorus, microbial biomass
carbon and phosphorus and phospholipids fatty acids profiles during 0, 6, 15,
20, 35, 45 and 60 days incubation period. While in the incubation experiment 1b
(mixing different rates of urea with fixed quantity of biosolids in soil), soil from
the same grassland, were being sampled at 10 day intervals (10, 20, 30, 40, 50,
and 60) after taking the initial sample of day 0 and analysed for pH,
mineralisable P and phosphomonoesterase enzyme activities.
The incubation experiments showed that, mineralisation of available phosphorus
were significantly (p<0.001) higher in the biosolids and organomineral (OMF)
fertiliser treated soils compared to control during 20 and 30 days period. The
mean values of mineralisable P from OMF treatment for the incubation
experiment 1a were 14.5 and 19.5 mg/kg in the grass and arable soils
respectively. The mineralisation rates of biosolids-P from organomineral
fertiliser (OMF) amended soils were also reduced significantly, as the dosages
of urea component were increased according to the 50, 150 and 250Kg/N
equivalents during the 60 days incubation experiment 1b. The mean available P
values from the OMF amendments were 28.0, 25.7 and 23.4mg/kg respectively;
according to the increasing amount of urea content at 50, 150 and 250Kg/N
equivalents respectively. The overall trend of organomineral fertiliser
phosphorus (OMF-P) mineralisation rate have shown significant (p<0.001)
reduction with increased doses of urea components, with the fixed biosolids
equivalent rate of 250Kg/ha during the incubation experiment 1b. There were
no significant (p>0.01) changes in pH from both soil incubation experiment 1a
and 1b, except for the urea only treated samples at the 250KgN/ha equivalent
rate from the sandy clay loam grassland soil, which shows up to 1 unit increase
(pH=8.1) compared to other treatments, just a day after kick-starting the
incubation experiment, but eventually becomes reduced to the original pH (6.9)
during the incubation period. Microbial community change in both arable and grassland soil from initial day
zero showed distinct and consistent shifts in trends through the 20 and 45 days
respectively, irrespective of their treatments, and then gradually shifted towards
the original starting point at the final incubation study of day 60. This was
however attributed to the function of change with time, since it could not be
categorically assigned to the OMF application effects only, but perhaps
community change effects with time could be the main factor. Even though there
was no any important patterns or trends observed between the indigenous
treatments, but the wide spreading and shift distances amongst treatments
during 20 and 45 days were higher compared to the 0 and 60 days, and this is
probably because there were more phosphorus mineralisation when
microorganisms were able to access more dissolved organic carbon as shown
by high biomass carbon during 20 and 35 days incubation time, and utilise it to
generate energy that kept them more active within those period before it
gradually becomes exhausted, since there was no external source of energy
being added. Similarly, phosphomonoesterase enzyme activities in the soil
treatments except for the urea only amended samples, showed significant
(p<0.01) differences between days 20, 30 and 40 compared to days 0, 50 and
60 and the phosphatase activities in the OMF amendments had significantly
higher acid than alkaline phosphatase activities. Organomineral phosphorus
(OMF-P) mineralisation in soil during 60 days incubation in this short-term study
have shown potential P release in both soils, and the OMF-P mineralisation rate
was highest in the formulation having fixed biosolids with urea at 150KgN/ha
equivalent compared to other formulations (50KgN/ha and 250KgN/ha) and
therefore effects of urea component of the organomineral fertiliser is an
important factor when considering OMF as a promising P alternative or source
in low-P soil during phosphorus management. Appropriate product formulation
depending on the crop needs is therefore very important for soil phosphorus
nutrient management and sustainability.