Effect of pearl millet–cowpea cropping systems on nitrogen recovery, nitrogen use efficiency and biological fixation using the ¹⁵N tracer technique

Figures & data

Table 1 Nitrogen (%) and 15N atom% excess of the plant components

Plant components	Cropping system	Dry matter (kg ha^−1)	Nitrogen (%)	^15N atom% excess
Grain/seed	SM	1135	2.31a	0.139a
	IM	570	1.93a	0.112a
	SC	1444	3.72a	0.508b
	IC	1778	3.69a	0.458b
	P	0.087	< 0.001	< 0.001
	LSD0.05	941	0.5	0.06
Straw/herbage	SM	3792a	1.38	0.158a
	IM	2750b	1.28	0.135a
	SC	2117b	1.32	0.405b
	IC	1976b	1.31	0.370b
	P	0.012	0.983	< 0.001
	LSD0.05	945	0.64	0.07
Ears/pods	SM	1115	1.74	0.141a
	IM	459	1.47	0.121a
	SC	1095	1.43	0.527b
	IC	1364	1.48	0.419c
	P	0.184	0.451	< 0.001
	LSD0.05	894	0.49	0.08
Within a column means followed by a common letter are not significantly different at P < 0.05. IC, intercrop cowpea; IM, intercrop millet; LSD, least significant difference; SC, sole cowpea; SM, sole millet. Grain, straw and ears are from millet and seed, herbage and pods are from cowpea.

Table 2 Nitrogen recovery, nitrogen fertilizer use efficiency and N2 fixation (15N dilution method) of cowpea as related to the cropping system

Cropping system	DM	TN	N recovery	Ndff	NUE	Ndfs	Ndfa
	(kg ha^−1)			(%)
SM	6041	95.26a	14.46	15.30a	35.28a	84.71	–
IM	3779	53.23b	6.23	12.56ab	15.20b	87.43	–
SC	4656	98.46a	9.06	9.40bc	45.88a	66.93	23.66
IC	5118	110.73a	9.21	8.46c	46.00a	63.21	28.33
P	0.072	0.018	0.084	0.019	0.003	0.259	0.148
LSD0.05	1644	30.91	3.45	3.91	12.03	32.24	32.52
Within a column means followed by a common letter are not significantly different at P < 0.05. DM, dry matter; IC, intercrop cowpea; IM, intercrop millet; LSD, least significant difference; N recovery, amount of nitrogen in plants derived from the fertilizer; Ndfa, percentage of nitrogen derived from atmosphere (N2 fixation); Ndff, percentage of nitrogen derived from the fertilizer; Ndfs, percentage of nitrogen derived from soil; NUE, nitrogen (fertilizer) use efficiency; SC, sole cowpea; SM, sole millet; TN, total nitrogen. Applied N: 41 kg ha^−1 in millet and 20 kg ha^−1 in cowpea. ^15N atom% excess of fertilizer: 0.984% in sole millet, 1.024% in intercrop millet, 5.107% in sole and intercrop cowpea, 1.024% in reference (a millet crop) sole cowpea. Intercrop millet is the non-fixing reference plant for intercrop cowpea. Sole cowpea Ndffref = 12.46% (calculation not shown) Ndff (%) = (^15N atom% excess of plant sample/^15N atom% excess of fertilizer) × 100. Ndfa (%) = 1 – (Ndfffix/Ndffref) × 100. Ndfffix is the Ndff (%) of the N-fixing plant (cowpea) and Ndffref the Ndff (%) of the non-ixing reference plant (millet). Ndfs (%) = 100 – Ndff (%) for millet crop. Ndfs (%) = 100 – Ndff (%) – Ndfa (%) for cowpea. TN (kg ha^−1) = (DM × N%) × 100. NUE (%) = (N Recovery/ Applied N)  × 100.

Table 3 Nitrogen contents and 15N atom% excess in 0–10 and 10–20 cm soil depths

Soil depth (cm)	Cropping system	Nitrogen (%)	TN (kg ha^−1)	^15N atom% excess	Ndff (%)	Ndff (kg ha^−1)
0–10	SM	0.056	850.00	0.020	2.00a	16.97a
	SC	0.043	650.00	0.029	0.57b	3.77b
	INT	0.045	825.00	0.027	0.55b	4.15b
	P	0.351	0.351	0.751	0.011	0.008
	LSD0.05	0.022	339.79	0.031	0.80	6.74
10–20	SM	0.043	650.00	0.010	1.00	6.77
	SC	0.043	650.00	0.020	0.40	2.67
	INT	0.04	600.00	0.011	1.05	1.20
	P	0.884	0.884	0.072	0.114	0.067
	LSD0.05	0.017	256.86	0.009	0.68	4.44
Within a column means followed by a common letter are not significantly different at P < 0.05. INT, Millet–cowpea intercrop; SC, sole cowpea; SM, sole millet; TN, total nitrogen. Ndff (%) and Ndff (kg ha^−1) are the percentage and the amount of N fertilizer in the total nitrogen of the soil, respectively. Ndff (%) = (^15N atom% excess of soil sample/^15N atom% excess of fertilizer)  × 100. Ndff (kg ha^−1) = (TN × Ndff (%))/100.

Table 4 Nitrogen immobilization in 0–20 cm soil depth and nitrogen loss from nitrogen fertilizer after the experiment

Cropping system	TN	Ndff	Nitrogen immobilized	Nitrogen loss
	(kg ha^−1)		(%)
SM soil	1483.33	23.73a	57.88a	6.84
SC soil	1300.00	6.43a	32.16b	21.96
INT soil	1425.00	5.45a	27.09b	11.71
P	0.644	0.001	0.034	0.078
LSD0.05	485.47	6.17	21.25	20.53
Within a column means followed by a common letter are not significantly different at P < 0.05. INT soil, millet–cowpea intercrop soil; SC soil, sole cowpea soil; SM soil, sole millet soil; TN, total nitrogen; Ndff = amount of nitrogen fertilizer in the total nitrogen of the soil. Nitrogen immobilized = percentage of nitrogen fertilizer recovered in the soil after cultivation. Nitrogen loss = percentage of nitrogen fertilizer lost during the experiment. Nitrogen immobilized = (Ndff (kg ha^−1)/Applied N (kg ha^−1)) × 100. Nitrogen loss = 100 – nitrogen immobilized (%) – NUE (%). NUE (%) is the nitrogen use efficiency of the crops in Table 2.

Table 5 Land Equivalent Ratio (LER) of the experiment based on grain and total dry matter

Cropping system	Grain yield (kg ha^−1)	Total dry matter (kg ha^−1)
Intercrop millet	508	3688
Sole millet	1135	6041
Intercrop cowpea	1364	5118
Sole cowpea	1444	4653
LER	1.68	1.71
LER = (intercrop millet yield/sole millet yield) + (intercrop cowpea yield/sole cowpea yield). The intercrop presents more benefit when LER > 1 and the sole crop is more advantageous when LER < 1.