9.2.1 Litter-fall: significant quantities in intensively grazed dairy pastures
The rationale for this study came from observing grazing dairy cattle dropping freshly harvested plant material onto the soil surface, hereafter called litter-fall. For the first time, this study (Chapter 4) quantified litter-fall in intensively grazed dairy pastures. During grazing the fresh litter-fall rate was 53 ± 24 kg DM ha–1 per grazing event. This equated to an annual N application rate of 15.9 kg N ha–1 y–1 and 3.5 kg N ha–1 y–1 for fresh and senesced litter, respectively. The amount of N contained in the annual litter-fall rate was comparable in
magnitude to a typical fertiliser application rate (~25 kg N ha–1). Litter-fall accounted for 4% of the apparent dry matter consumption of the dairy cattle. This has implications for dry matter budgeting i.e. budgets not accounting for litter-fall may overestimate DMI of the animals. Since this litter contained N, it was hypothesised that during its decomposition it could contribute to N cycling and N2O emissions. The litter-fall rates measured also raised
further questions.
Would grazing management change litter-fall rates? For example, pasture utilisation, stocking rate, pasture and animal species, climate, grazing intensity, could all
potentially affect litter-fall.
Could litter-fall at the above measured rates contribute to N2O emissions?
If so, are N2O emissions affected by the biochemical composition of the litter?
If litter-fall contributes to N2O emissions, are there implications for the IPCC
methodologies or assumptions?
The answers to some of these questions were obtained in this current work.
9.2.2 Animal treading increases N2O emissions irrespective of the presence of
herbage
Approximately 20% of the world’s pastures and rangelands are considered degraded through overgrazing and compaction (Steinfeld et al. 2006). The magnitude of compaction depends on the stocking rate, soil type, moisture content and animal species (Naeth et al. 1990; Warren et al. 1986). Chapter 5 investigated the effect of partial incorporation of pasture herbage due to animal treading on N2O emissions. Results showed that:
Treading lowered soil NO3––N concentrations and increased N2O emissions,
irrespective of the presence or absence of herbage indicating utilisation of NO3– by
N2O producing microorganisms. In Chapter 5 (part B), treading diluted the 15N
enrichment of the soil NO3– pool presumably due to the release of unlabelled soil-N
and/or herbage-N.
The suppression of the CO2 emissions due to treading (parts A and B) indicated an
enhancement of anaerobic conditions in the trodden plots thereby increasing the chances of denitrification contributing to N2O emissions.
To further understand the effect of herbage treading on N2O emissions given the
results of Chapter 5 and other work (Chapter 8) in this thesis, 15N labelled herbage should be used in further studies to determine the extent of the increase in the soil inorganic N pool following treading.
9.2.3 Pasture litter – a significant, anthropogenic N2O source
Studies have shown that N2O emissions occur from incorporation of crop residues into
soil (Section 2.4) and the magnitude of these emissions has been shown to depend on the biochemical composition, rate and placement of litter into the soil (Section 2.4). Chapter 4 showed that litter-fall was significant in pasture. Chapter 6 was a laboratory study to quantify N2O emissions using two pasture species and a winter supplement that dominate in New
Zealand. Ground shoots of clover, ryegrass and maize were incorporated into soil. Results showed that:
Maximum N2O emissions occurred relatively rapidly (0.5 d) after litter incorporation
indicating rapid mineralisation of plant litter-N and subsequent utilisation by nitrifiers and/or denitrifiers.
Emission factors (EF) for N2O equated to 2–3% of the incorporated N at 86% WFPS
while at 54% WFPS, EF was significantly less with 1.7% > 0.7% = 0.5% for clover, ryegrass and maize, respectively; these differences between species were attributed to the biochemical properties of the litter species including their differing C: N ratios. Cumulative emissions from these, albeit unrealistically high rates of litter-N
incorporation ranged from 63–209 kg N2O-N ha–1 y–1 at 54% WFPS and 269–359 kg
N2O-N ha–1 y–1 at 86% WFPS, respectively. The significant N2O emissions, especially
for clover and ryegrass, warranted further study (Chapters 7 and 8). Combining the EF results and the in situ litter-fall data, it was estimated that N2O emissions, attributable
to litter-fall alone, could be 0.4 kg N2O ha–1 y–1, which is similar to the reported values
of ‘background’ emissions from grazed pasture soils (Section 2.2.5). But unlike these so called ‘background’ emissions, these litter-fall-derived N2O emissions are clearly
anthropogenic and therefore should be acknowledged and accounted for in the IPCC methodology.
9.2.4 Biochemical composition of litter: effect of cellulose on N2O emissions
Results from Chapter 6 showed that the biochemical composition of the litter could determine the N2O emissions. Clover had a lower C: N ratio (of 9) and lower cellulose and
hemicellulose contents. To investigate the role of the C: N ratio with respect to N2O
emissions, increasing amounts of cellulose were incorporated with a constant amount of clover litter (in Chapter 7) since it was rationalised that the lower C: N ratio of the clover litter in Chapter 6 may have meant a lack of a C substrate for denitrifiers to complete the reduction of N2O to N2. Results showed that:
Clover incorporation into soil rapidly produced N2O emissions and adding increasing
quantities of cellulose significantly enhanced those N2O emissions, indicating that the
incorporated cellulose acted as a labile C source for the heterotrophic organisms responsible for denitrification.
Increases in soil inorganic N concentrations from the treatments indicated that ammonification of the plant litter occurred followed by nitrification of the NH4+ to
NO3– and its further consumption in the presence of the added C substrate i.e.
cellulose.
Over 42 d, 50–90% of the total N2O was emitted in ~9 d from the cellulose-amended
treatments. An important implication from this study was that higher C: N ratios did not necessarily mean that the material was recalcitrant with respect to decomposition and N2O emissions; rather it was the biochemical composition, relative concentrations
and the form of the recalcitrant compounds in the plant litter which played key roles. Emissions of N2O increased with increasing C: N ratio of the litter and cellulose
combinations.
Further investigation should be performed using 15
N-labelled plant materials to further establish associated N2O emissions and N cycling during their decomposition.
9.2.5 Surface-placed litter stimulates N2O emissions
In Chapter 8, 15N-labelled ryegrass was placed on the surface of a pastoral soil in litterbags at an unrealistically high rate of 213 kg N ha–1 and N2O and CO2 emissions were
measured. This study is the first study that reports soil N dynamics and N2O emissions using a 15
N technique with pasture litter in situ. Results showed that:
Approximately 70% of the N2O came from the shoots with peak N2O emissions 5 to
10 d after litter placement.
Emissions of N2O may have resulted from ammonification followed by a coupling of
nitrification and denitrification during litter decomposition on the soil surface. The 15N enrichment of soil inorganic N (NO3–) and the evolved N2O demonstrated that the
emissions originated from litter-N. These processes supported the hypotheses established in Chapters 6 and 7.
The EF of the in situ placed litter was 0.9%. This result in conjunction with the litter- fall rates measured (Chapter 4) has implications for background N2O emissions and