FEATURE
Table 1 : The predicted annual N 2 O emissions are based on the Intergovernmental Panel on Climate Change ( IPCC ) emission factors ( EFs ) ( IPCC min
+ IPCC min + exc ). The EFs are calculated based on the N 2
O measurements ( EF data
) and linear ( E1 – E2 ) and nonlinear ( E3 – E4 ) regression approaches for the different N fertilization treatments ( N0 , N20 , N40 , N60 , and N80 expressed as kg N 2
O-N ha −1 year −1 ). The R square ( R 2 ), root mean square error ( RMSE ), and the Nash – Sutcliffe model efficiency coefficient ( NSE ) are shown as a result of the comparison between measured and predicted annual N 2
O emissions indicating the best fit of measured vs . predicted values . The N0 , N20 , N40 , N60 , and N80 refer to the different mineral fertilizer application rates used and were 0 , 220 , 440 , 660 , and 880 kg N ha −1 year −1 , respectively .
Treatment |
N0 |
N20 |
N40 |
N60 |
N80 |
R2 |
RMSE |
NSE |
IPCC min
|
0.0 |
2.2 |
4.4 |
6.6 |
8.8 |
0.59 |
3.15 |
0.30 |
IPCC min + exc
|
9.00 |
11.2 |
13.4 |
15.6 |
17.8 |
0.62 |
3.05 |
−1.07 |
EFdata |
0.0 |
1.9 |
3.8 |
5.8 |
7.7 |
0.59 |
3.15 |
0.11 |
E1 |
1.1 |
4.0 |
6.9 |
9.7 |
12.6 |
0.62 |
3.05 |
0.64 |
E2 |
1.2 |
4.2 |
7.2 |
9.6 |
11.9 |
0.56 |
3.30 |
0.58 |
E3 |
3.3 |
3.6 |
4.5 |
7.3 |
15.3 |
0.77 |
2.36 |
0.72 |
E4 |
2.7 |
3.6 |
5.4 |
8.4 |
13.3 |
0.71 |
2.65 |
0.63 |
Estimated methane emissions resulting from ruminal enteric fermentation were on average the largest contributor ( 49 %) to the total GWP per hectare over all treatments . More than half of emissions in the N60 and N80 treatments were the result of direct N 2
O emissions from mineral fertilizer applied to pastures and as a result of irrigation . Mineral N fertilizers as inputs accounted for the third largest contributor ( 12 %) of total GWP per hectare . Soil carbon sequestration had a positive effect in reducing the total GWP per hectare over all treatments ( Table 2 ).
Table 2 : The potential average milk yields , GWP , CF and N-balance for the various treatments ( N0 , N20 , N40 , N60 and N80 ) over the three years . SEM are shown in brackets .
Parameter
Treatment
N0 N20 N40 N60 N80
Milk yield ( t ECM ha −1 ) 14.8 a ( 0.7 ) 16.6 b ( 1.1 ) 16.5 c ( 0.9 ) 17.5 d ( 1.3 ) 18.0 e ( 0.9 )
GWP ( t CO 2 eq ha−1 ) 22.1 a ( 0.3 ) 25.3 b ( 0.4 ) 29.6 c ( 0.4 ) 36.6 d ( 0.1 ) 50.7 e ( 0.3 )
GWP + soil carbon ( t CO 2 eq ha −1 ) 18.4 a ( 0.2 ) 21.2 b ( 0.2 ) 25.4 c ( 0.2 ) 32.2 d ( 0.2 ) 46.2 e ( 0.5 )
CF + soil carbon ( kg CO 2 eq kg ECM −1 ) 1.3 a ( 0.1 ) 1.3 a ( 0.1 ) 1.6 b ( 0.1 ) 1.9 c ( 0.2 ) 2.6 d ( 0.2 )
Farm-N-balance ( kg N ha −1 ) 31 a ( 1.8 ) 241 b ( 3.8 ) 462 c ( 2.6 ) 677 d ( 5.0 ) 899 e ( 5.3 )
abcde no common lower-case letters indicate significant differences between treatments
With increasing farm-N-balance per hectare , the GHG emissions were generally higher . This relationship could best be described as exponential . In contrast , the ECM ha −1 correlated linearly with the farm-N-balance . However , the lowest farm-N-balance ( N0 treatment ) did not result in the lowest CF and were similar between the N0 and N20 treatment .
The results from the current study indicated an improvement in the carbon footprint as the amount of N-fertilizer was reduced . The main source of N came from purchased mineral fertilizers . Consequently , the fertilizer and the extensively imported supplements were the most prominent factors influencing the farm-N-balance , respectively . In South African pasture-based dairy systems , cows graze pastures year-round , which makes high N returns to pastures more likely . Dairy cows excrete ~ 75 % of their N intake , whereas less than ~ 25 % is metabolized into the milk output from the pasture system ( Luo & Kelliher 2010 ). Even if concentrates are fed at low levels there will still be a considerable amount of N returned through excreta . Therefore , excreta should be considered in fertilization management strategies . The CF of milk from pasturebased dairy farms can be further improved through management such as timing of fertilizer application , reducing the amount of fertilizer applied , and by incorporating forage legumes .
13 Grassroots Vol 21 No 3 November 2021