A nitrogen balance model for environmental accountability in cropping systems

Figures & data

Fig. 1  Flowchart of the proposed OVCrop module for N leaching estimation.

Fig. 2  (a) Temperature factor (f _TEMP ) for a range of mean monthly temperatures used in OVCrop. Dashed line is the scaled f _TEMP used in the calculation of denitrification. (b) The soil organic matter mineralization factor (f _SOM ) over time for two values of years in pasture (τ _YiP ).

Table 1  Soil water storage (mm) of the generic soils used in the simulations to derive the parameter values of OVCrop

	Soil texture
Depth of soil up to the gravel layer (m)	Light	Medium	Heavy
0.20	64	84	80
0.50	70	120	110
1.00	80	180	160
1.50	90	240	210

Fig. 3  Relative crop cover (a) and relative biomass accumulation (b) factors in relation to cumulative thermal time for spring and autumn wheat. Lines represent the model used in OVCrop.

Fig. 4  Relationship between the potential evapotranspiration and the soil evaporation simulated by LUCI over 25 years under bare soil conditions, for three different months. The dotted lines have the intercept as the origin and slope representing k _EVAPO . The continuous line represents the 1:1 relation.

Table 2  Analysis of variance for the main factors influencing evaporation in months when evaporation demand was higher than water inputs. The response variable was W _EVAPO /E _TP ^a

	df^b	SS^c	MS^d	F test	%Var^e
Site	3	254521	84840	2295.1	8.1
Soil texture	2	36694	18347	496.3	1.2
Soil depth	3	5397	1799	48.7	0.2
Irrigation	2	134076	67038	1813.5	4.3
Month	11	2431813	221074	5980.4	77.3
Residuals	6856	283328	41		9.1
^a W EVAPO is the monthly evaporation rate and E TP is the potential evapotranspiration; ^bDegrees of freedom; ^cSum of squares; ^dMean squares; ^ePercent of variation explained

Fig. 5  (a) Relationship between drainage and N leaching estimated by the LUCI model for the Lincoln site and two soil types over a series of scenarios (see text for details). (b) the relationship, on a logarithmic scale, between the drainage scaled to the soil water storage at field capacity and the ratio between N _LEACH (leaching) and N _LEACHEABLE (the sum of soil mineral nitrogen at the start of the month and nitrogen inputs and removals during the month).

Table 3  Analysis of variance for the main factors influencing N leaching. The response variable was set as log(N _LEACH /N _LEACHABLE )^a

	Df^b	SS^c	MS^d	F test	%Var^e
Log(W DRAIN /θ FC )	1	89914	89914	828046.9	86.85
Soil texture	3	9878	3293	30321.9	9.54
Soil depth	3	124	41	381.3	0.12
Site	3	17	6	51.6	0.02
Crop	3	193	64	593.1	0.19
Month	11	305	28	255.3	0.29
Residuals	28551	3100	0.109		2.99
^a N LEACH is nitrogen leaching and N LEACHABLE is the sum of soil mineral nitrogen at the start of the month and nitrogen inputs and removals during the month; ^bDegrees of freedom; ^cSum of squares; ^dMean squares; ^ePercent of variation explained

Table 4  Time line of the two crop rotation treatments (R1 and R2) during two seasons (S1 and S2) of the experiment used in the model evaluation

Year	2004			2005												2006												2007
Month	Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Jan	Feb	Mar
Rotation 1	Potato						Fallow					Peas					Fallow								Potato
			∣←——R1–S1——→∣∣←——R1–S2——→∣
Rotation 2	Potato						Wheat										Fallow								Potato
			∣←——R2–S1——→∣∣←——R2–S2——→∣

Fig. 6  Comparison of the annual estimates of drainage and N leaching under bare soil obtained using the OVCrop module and the LUCI framework for three sites in New Zealand. Points from each site represent a range of treatments and years. Diagonal line is the 1:1.

Table 5  Measures of agreement between annual values of evapotranspiration, drainage, change in soil mineral N, and N leaching from simulations of bare soil and crop rotations using OVCrop and the LUCI model

	Evapotransp. (mm yr^−1)	Drainage (mm yr^−1)	Mineral N (kg ha^−1 yr^−1)	N uptake (kg ha^−1 yr^−1)	N leaching (kg ha^−1 yr^−1)
Bare soil
General bias (%)	2.2	−3.2	1.6	–	−1.8
Mean abs. error	68.8	77.9	14.0	–	14.0
R ^2	0.612	0.854	0.952	–	0.899
Crop rotations
General bias (%)	0.4	−5.7	158.5	12.9	−11.9
Mean abs. error	56.9	65.7	23.9	50.2	10.5
R ^2	0.659	0.877	0.782	0.655	0.830

Fig. 7  Comparison between annual values of drainage and N leaching under crop rotation estimated by OVCrop and the LUCI framework for three sites in New Zealand. Points from each site represent a range of treatments and years. Diagonal line is the 1:1.

Fig. 8  Variability of N leaching predicted by the LUCI model over 25 years (boxplots) and the predictions by the OVCrop module using average weather data (bars) for three sites and two soil types. Data are from a series of spring wheat scenarios with different irrigation and fertilizer levels (see text for details).

Fig. 9  Comparison between the observed and predicted values of drainage and leaching for two crop rotations (R1 and R2) and two seasons (S1 and S2). Also are shown the values for R2–S2 after adjust for water drainage (see text and Table 4 for details). Diagonal line is the 1:1.