Nitrogen (N) inputs and outputs were measured over 3 years in a
trial with four farmlets (each with
16 randomly-allocated 0·4 ha paddocks) on permanent white clover/ryegrass
pastures which were
grazed throughout the year by dairy cows near Hamilton, New Zealand. Three
farmlets were stocked
at 3·3 cows/ha and received nominal rates of N fertilizer (urea
in 8–10 split applications) of 0, 200
or 400 kg N/ha per year. A fourth farmlet with 4·4 cows/ha
received 400 kg N/ha per year and was
supplemented with maize grain during the first two years.
Nitrogen balances were calculated, with [sum ]N inputs[ape ][sum ]N outputs.
Annual inputs from N2 fixation
were 99–231 kg N/ha in the 0 N farmlet, but declined to 15–44
kg N/ha in the 400 N farmlets. The
main N outputs (in kg N/ha per year) were in milk (72–126), nitrate
leaching (20–204), and transfer
of N via cow excreta from pastures to lanes and milking shed (54–92).
Gaseous losses by
denitrification (3–34) and volatilization (15–78) were smaller
than the other N outputs but increased
significantly with N fertilizer application. In the maize-supplemented
farmlet, N outputs in milk were
31% higher than in the corresponding non-supplemented 400 N farmlet, whereas
averaged 17% lower during the 2 years of supplementation.
In the N-fertilized farmlets, estimated N balances were influenced by
inclusion of the transitional
N processes of immobilization of fertilizer N into the soil organic N pool
42–94 kg N/ha per year) and the contribution from mineralization
of residual clover-fixed N in soil
not accounted for in the current estimates of N2 fixation (estimated
at up to 70% of measured N2
fixation or 46 kg N/ha per year). However, these processes were counteracting
and together were
calculated to have only a small net effect on total N balances.
The output of N in products (milk, meat and feed) relative to the total
N input averaged 26% in
the 400 N farmlets, and is compared to that measured for commercial intensively-managed
farms in England and the Netherlands (14–20%). The 0 N farmlet, which
was reliant on N2 fixation
as the sole N input, was relatively very N-efficient with the milk production
being 83% of that in the
400 N farmlet (at 3·3 cows/ha) and the N output in products
relative to total N input averaging 52%.