FEATURE
Lolium perenne ) pastures . Plots were 15 × 15 m and replicated in 4 blocks . The study was conducted near the city of George in the southern Cape region of South Africa ( 33 ° 58 ' 38 ” S ; 22 ° 25 ' 16 ” E ; 201 m . a . s . l .). N 2
O emissions were captured using the static chamber method ( Hutchinson & Mosier , 1981 ). Gas measurements were performed for one year on a weekly basis unless in the case of a fertilization event , where it was done in three consecutive days after fertilization . Gas samples were analysed for
N 2
O through a gas chromatograph to determine daily fluxes ( Smit et al . 2020 ). Cumulative fluxes were calculated by means of linear interpolation . Furthermore , three-year ( April 2016 to June 2019 ) experimental field data was used to investigate the effect of mineral fertilizer levels , as management strategies , on the pasture yield and the PCF of produced milk . The additional N-excretion from grazing animals was considered and was calculated to be evenly distributed at 450 kg N ha -1 year -1 for all treatments . Forage quality parameters were estimated using near-infrared reflectance spectroscopy ( NIRS ). The onfarm soil organic carbon ( SOC ) changes of the tested production systems were also estimated . The global warming potential ( GWP ) per hectare was calculated using the respective value for each trace gas ( CO 2
= 1 , N 2 O = 265 , CH 4
= 28 ) over a lifespan of 100 years ( IPCC 2014 ) and expressed as CO 2 eq . The efficiency of the different N fertilization strategies ,
in relation to climate change , was calculated on the basis of the functional unit ECM as proposed by Sjaunja et al . ( 1990 ). The farm-N-balance was calculated using a simple equation that deducts the nitrogen outputs at the farm gate from the sum of the nitrogen inputs .
Results and Discussion Accumulated N 2
O emissions ranged between 2.45 and 15.5 kg N 2
O-N ha -1 year -1 ( Figure 1 ) and EFs for mineral fertilizers applied had an average of 0.9 %. Therefore , the IPCC default value EF for N-deposition from animal excreta seems to be overestimated .
Figure 1 : Accumulated N 2
O-N emissions for the different N fertilization treatments ( N0 , N20 , N40 , N60 , and N80 ) over the trial period . Error bars denote standard error of the mean . Different letters indicate significant differences between the treatments ( p < 0.05 ). The N0 , N20 , N40 , N60 , and N80 refer to the fertilizer rates used as treatments and were 0 , 220 , 440 , 660 , and 880 kg N ha −1 year −1 , respectively .
Figure 2 : The linear relationship between accumulated N 2 O-N losses ( kg N 2
O-N ha −1 year −1 ) in relation to ( E1 ) increased levels of N-input ( kg N ha −1 year−1 ) as well as ( E2 ) increased levels of N balance ( kg N ha −1 year −1 ). The nonlinear relationship between accumulated N 2
O-N losses ( kg N 2
O-N ha −1 year −1 ) in relation to ( E3 ) increased levels of N-input ( kg N ha −1 year −1 ) as well as ( E4 ) increased levels of N balance ( kg N ha −1 year −1 ).
The relationship between N 2
O-N losses and N input can best be described by an exponential function ( Figure 2 ) rather than a linear function , which indicated that excessive fertilization of N will add directly to N 2
O emissions from the pastures . There was no positive effect on the growth of pasture herbage from adding N at high rates . The suggested EFs of the IPCC default value for grazing systems led to an overestimation of N 2
O emissions ( Table 1 ) when they were compared to the estimated values obtained from the current study . A better approach would be to replace EFs of the IPCC default value with regional EF values , which are dependent on the N balance . This leads to more accurate greenhouse gas inventories from managed soils on a regional scale , where other environmental threats ( e . g . groundwater pollution and eutrophication ) are also addressed .
Grassroots Vol 21 No 3 November 2021 12