Availability of nitrogen in green cane trash blanketed soils in the wet tropics and its impact on productivity / profitability: a systems analysis

Mrs Elizabeth Meier (2007). Availability of nitrogen in green cane trash blanketed soils in the wet tropics and its impact on productivity / profitability: a systems analysis PhD Thesis, School of Natural and Rural Systems Management, The University of Queensland.

       
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Author Mrs Elizabeth Meier
Thesis Title Availability of nitrogen in green cane trash blanketed soils in the wet tropics and its impact on productivity / profitability: a systems analysis
School, Centre or Institute School of Natural and Rural Systems Management
Institution The University of Queensland
Publication date 2007-12
Thesis type PhD Thesis
Supervisor Dr Malcolm Wegener
Professor Kaye Basford
Dr Peter Thorburn
Total pages 231
Total colour pages 24
Total black and white pages 207
Subjects 300000 Agricultural, Veterinary and Environmental Sciences
Formatted abstract Sugarcane crops residues (called ‘trash’) contain 30-50 kg nitrogen (N)/ha. Sugarcane
crops are commonly burnt before harvesting to facilitate that operation. However,
there is a growing trend towards the cessation of burning in many sugarcane
producing countries. The wet tropics region of far north-eastern Australia was one of
the first regions where cane is harvested mechanically to move away from burning –
trash has been retained after each crop for up to 30 years. The impact of long term
trash retention on the N content of wet tropical soils is not fully understood, as high
rainfall increases potential N losses by denitrification, leaching and runoff. The
purpose of this thesis was therefore to investigate the effect of retaining trash on the N
content of sugarcane soils in the Australian wet tropics, and the implications of such
changes for productivity and profitability. A range of techniques was used in this
investigation, including field studies of N dynamics using 15N labelled trash and N
fertiliser, laboratory studies of nitrification processes, and analyses of different
management practices with a cropping systems simulation model (APSIM).
In the field study, only a small proportion (< 6%) of the N contained in trash or
applied as fertiliser at the beginning of the field experiment was recovered in the first
or second crop after treatments were applied. Most N from applied trash and fertiliser
was recovered in the 0-1.5 m soil layer, with there also being some evidence of
movement of N below this depth.
Surprisingly, ammonium-N (NH4
+-N) concentrations frequently exceeded nitrate-N
(NO3
--N) in the soils even though soil conditions (e.g. pH and soil water content)
were conducive to nitrification. So laboratory incubation experiments were also
conducted to investigate whether nitrification was impeded. NO3
--N increased
significantly during incubations of field experimental soils. Virtually all NH4
+-N was
nitrified during the incubation period, and nitrification was not limited by NH4
+-N
supply, nitrifier population or high soil moisture content. The dynamics of NH4
+-N
and NO3
--N were simulated with the SoilN2 module of the APSIM cropping systems
model. Coefficients for nitrification in SoilN2 were reduced by two-thirds to improve
accuracy of simulated net nitrification and ammonification in the incubated soils.For the simulation analysis of different management practices, the APSIM cropping
systems model was parameterised with soil and sugarcane crop data from the
experimental sites. Predictions of cane yield and changes in soil N and carbon (C)
from the APSIM cropping systems model were validated with three data sets of
increasing independence from the field experiment microplots: (1) block data, (2)
changes in soil C and N since the farms were first cultivated, and (3) four independent
field experiments conducted in the region. The validated model was then used to
simulate a factorial combination of trash management practices and N fertiliser rates
on four different soils in two climates. These climates were selected to represent
examples of the humid (~2000 mm rainfall/y) and super-humid (~4000 mm rainfall/y)
extremes of canegrowing environments in the wet tropics. In these scenarios, yield
was not affected by trash management or N fertiliser rates greater than 60 kg N/ha
(plant crops) and 80 kg N/ha (ratoon crops) within all combinations of soil and
climate. Total N losses from the soil-plant system by combined NO3
- leaching,
denitrification and trash burning increased rapidly with increasing rates of N
application. Soil N concentrations were greater in trash-retained than in burnt trash
management systems. However, this difference was not reflected in the yield response
curves for N due to immobilisation of N fertiliser in the trash blanket and ‘luxury
uptake’ of N by the crop in the simulations. Average partial gross margins calculated
from the simulation information at all N rates were greater in burnt trash systems for
plant crops, but greater in ratoon crops when trash was retained. Although the cost of
N losses was a small proportion of the partial gross margins, N losses can cause
significant negative environmental effects. Consequently, partial gross margins
underestimate the full cost of the burnt trash systems.
It was concluded that N mineralisation from a single trash blanket was not important
for sugarcane production in the wet tropics, and that nitrification processes in the wet
tropical soils studied were not different from those in the subtropical soils. High soil
NH4
+-N concentrations appeared to be due to losses of NO3
--N following nitrification.
The low N use efficiency by crops from fertiliser and high soil N concentrations
indicated that N fertiliser rates might be reduced. Simulations supported a potential
halving of conventional N rates, due to factors associated with production in the wet
tropical environment rather than from trash-induced improvements in soil N fertility.
Additional Notes Page numbers that should be printed in colour: 12,17,54,55,95,96,136,137,148,150-154,156,157,161-163,165,166,170,171,175. Page numbers that are in landscape: 98,102-106,109-112.

 
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Created: Wed, 22 Oct 2008, 21:21:13 EST by Mrs Elizabeth Meier